Part 5 (1/2)
Accordingly, if we confine our study to the two contrasted characters, tallness and dwarfness, we see that just three kinds of peas exist, namely, dwarfs which breed true, talls which breed true, and talls which always give the same definite proportion of talls and dwarfs among their descendants. Innumerable experiments which have since been made with other pairs of characters have demonstrated that this same mathematical proportion holds good throughout the whole world of plants and animals;[25] and hence this astonis.h.i.+ng result is now called Mendel's Law, and is regarded as the most important discovery in biology in several generations.
[Footnote 25: When dealing with only a few individual cases, we do not always find them to come out in such exact proportion; but when the number of examples is large, the proportion is so close to these figures that the exceptions can be entirely neglected as probably due to error of some kind.]
There are two distinct kinds of Andalusian fowls, one pure bred black, the other pure bred white with slight dashes of black here and there.
When these are mated, no matter which color is the father or the mother, the next or hybrid generation are always a queer mixture of black and white called by fanciers blue. When these blues are interbred, one-quarter of their offspring will be white, which will prove to breed true ever afterwards, one-quarter will be black that will breed true, and fifty per cent. will be blue which will break up in the next generation in the very same way as before. In this case neither white nor black character is dominant, and accordingly we have a blending of both in the first hybrid generation.
In guinea pigs, black color has been found to be dominant over white, rough coat over smooth coat, and short hair over long hair. These remarkable results following from an experimental trial of Mendelism have stimulated hosts of investigators in all parts of the world, until now many varieties of plants and animals have been studied for many successive generations, already, building up a considerable literature dealing with the subject.
Perhaps the most extensive and exact series of experiments along this line have been carried on by Thomas Hunt Morgan and his a.s.sistants, of Columbia University. For over five years they have been breeding the wild fruit fly (_Drosophila ampelophila_), during which time they have originated and observed over a hundred and twenty-five new types that breed true according to Mendel's laws. Every part of the body has been affected by one or another of these mutations. The wings have been shortened, or changed in shape, or made to disappear entirely. The eyes have been changed in color or entirely eliminated. And each of these wonderful variations was brought about not gradually, but at _a single step_.
Professor Morgan grows justifiably sarcastic in contrasting these demonstrated laboratory facts with the armchair theories that have so long and so harmfully dominated biological studies. A quotation from him will not be out of place at this point.
”I may recall in this connection that wingless flies also arose in our cultures by a single mutation. We used to be told that wingless insects occurred on desert islands because those insects that had the best developed wings had been blown out to sea. Whether this is true or not, I will not pretend to say; but at any rate wingless insects may also arise, not through a slow process of elimination, but at a single step.... Formerly we were taught that eyeless animals arose in caves.
This case shows that they may also arise suddenly in gla.s.s milk bottles, by a change in a single factor.”[26]
[Footnote 26: ”A Critique of the Theory of Evolution,” p. 67.]
We need not be particularly concerned here with the theoretical explanations of these facts offered in terms of the microscopic or even the infra-microscopic components of the germ cells. Morgan seems to make out a strong case for the theory that the chromosomes found in the nucleus are the real ultimate units that carry the hereditary factors.
But he is quite decided in the opinion that these hereditary factors are fixed, and are not changed from generation to generation either by environment or by selection.[27] The important thing for us in this connection is to get a clear idea of the results following from an application of Mendel's laws to the old, old problem of the origin of species, incidentally noticing how the theory a.s.sociated with Darwin's name now looks in the light of these new facts.
[Footnote 27: In human beings it has been found that the effects of alcoholism and of syphilis are indeed transmitted according to Mendelian law, being the two solitary examples of diseased conditions that are thus transmitted. But they are so plainly pathologic phenomena that there is little temptation for the advocates of Lamarckianism to use them as proofs of their theory.]
We have hitherto been considering the results worked out by Mendel with but one pair of contrasted characters or factors. But Mendel studied the relation of other characters of the pea, and found among other results that smooth seeds are dominant to wrinkled seeds, colored seeds dominant to white, yellow color dominant to green, etc. But when a combination of _two_ factors in each parent are put into contrast by cross breeding, two wholly original forms (as they seemed) were sometimes produced, and it looked as if these new kinds were really a.n.a.logous to new species.
For example, he crossed tall yellow peas with dwarf green peas, with the result that the first hybrid generation turned out to be all tall yellows. However, in the second hybrid generation they split up according to the law as already stated, modified by the additional complication brought into the problem by the additional pair of factors.
For out of every sixteen plants there were nine tall yellows, three _dwarf yellows_, three _tall greens_, and one dwarf green. It is evident that these tall greens and dwarf yellows are really new forms; and further experiments proved that they can be separated out or segregated and grown as pure forms which thereafter breed true. Thus we have a very important result for the breeder, for it enables him to work to a definite aim and combine certain desirable characters into a single form.
The term _mutation_, as already intimated, has been given to this process of producing new varieties in this way. The kinds so produced are termed _mutants_, and at first they were hailed by enthusiastic scientists as ”elementary species.” De Vries in particular gave much publicity to this idea; for he thought he had really produced a new kind comparable in every respect to a true species as produced by nature among wild plants. But the enthusiasm with which this applied result of Mendel's Law was at first hailed by biologists has gradually subsided; for it has been found that though these new forms will breed true under certain conditions, they are nevertheless _cross-fertile with the original forms_, and thus the circle can be _completed back again_ by a return to the parent form, from which the new ”species” can again be produced at will with the same mathematical exactness as before.
III
Where then are we?
Clearly we have not really produced any new species in any correct sense of the word. If we have produced new forms that breed true and that are seemingly just as deserving of the rank of distinct species as many now listed in scientific books, it only shows that our lists are sadly at fault, and that they are not all species that are called species. These experiments merely indicate that _the parent form possesses more potential characters than it can give expression to in a single individual form_, some of them being necessarily latent or hidden, and that when these latent ones show themselves they must do so at the expense of others which become latent or hidden in their turn. This _vital elasticity_, as it may be termed, or the vital rebound under definite conditions, is indeed a prime characteristic of the species just as it is of the individual; but like that of the individual the vital elasticity of the species is strictly bounded by comparatively narrow limits beyond which we have never seen a single type pa.s.s under either natural or artificial conditions. Mutations can be made according to Mendel's Law; but when we have made them once _we can always be sure of producing the_ _very same mutants again in the very same way_, as surely as we produce a definite chemical compound; and when we have made it _we can always resolve it at will back into its original form_, just as we can a chemical compound. And so, where is the evolution? or how do these facts throw any light on the problem of the origin of species, any more than chemical compounds throw light on the origin of the elements?
Obviously in biology as in chemistry we are only working in a circle, merely marking time.
And the bearing of these facts on the other problem of the transmission of acquired characters is quite obvious. Mendelism provides no place for any such transmission. Mendel's Law is sometimes called the law of _alternative inheritance_, thus embodying in its name the thought that offspring may show the characters possessed by one parent or by the other, but that it cannot develop any characters whatever which were not manifest or latent in the ancestry. Changes in the environment during the embryonic stage, it is true, seem sometimes to be registered in the growing form; but it has never yet been proved that these induced changes can ever amount to a unit character or genetic factor that will maintain itself and segregate as a distinct factor after hybridization.
Ancestry alone furnishes the material for the factor, and no amount of induced change can get itself registered in the organism so as to come into this charmed circle of ancestral characters which alone seem to be pa.s.sed on to posterity.
A quotation from Bateson ought to set this point at rest:
”The essence of the Mendelian principle is very easily expressed. It is, first, that in great measure the properties of organisms are due to the presence of distinct, detachable elements [factors], separately transmitted in heredity; and secondly, that _the parent cannot pa.s.s on to offspring an element, and consequently the corresponding property, which it does not itself possess_.”[28]
[Footnote 28: _Scientific American_ Sup., January 3, 1914.]
Heredity we now see is a method of a.n.a.lysis, and the facts brought to light by Mendelism help us very much toward an understanding of living matter. Especially does it help us to understand the complexity underlying the facts of heredity, which until now have seemed so strange and capricious. As Professor Punnett of Cambridge remarks: