Part 7 (2/2)
73) that corresponds to the curve of probability (fig. 74).
[Ill.u.s.tration: FIG. 72. Beans put into cylindrical jars according to the sizes of the beans. The jars arranged according to size of contained beans.
(After de Vries.)]
[Ill.u.s.tration: FIG. 73. A curve resulting from arrangement of beans according to size. (After de Vries.)]
If we stand men in lines according to their height (fig. 75) we get a similar arrangement.
[Ill.u.s.tration: FIG. 74. Curve of probability.]
[Ill.u.s.tration: FIG. 75. Students arranged according to size. (After Blakeslee.)]
The differences in size shown by the individual beans or by the individual men are due in part to heredity, in part to the environment in which they have developed. This is a familiar fact of almost every-day observation. It is well shown in the following example. In figure 76 the two boys and the two varieties of corn, which they are holding, differ in height. The pedigrees of the boys (fig. 77) make it probable that their height is largely inherited and the two races of corn are known to belong to a tall and a short race respectively. Here, then, the chief effect or difference is due to heredity. On the other hand, if individuals of the same race develop in a favorable environment the result is different from the development in an unfavorable environment, as shown in figure 78. Here to the right the corn is crowded and in consequence dwarfed, while to the left the same kind of corn has had more room to develop and is taller.
[Ill.u.s.tration: FIG. 76. A short and a tall boy each holding a stalk of corn--one stalk of a race of short corn, the other of tall corn. (After Blakeslee.)]
[Ill.u.s.tration: FIG. 77. Pedigree of boys shown in Fig. 76. (After Blakeslee.)]
Darwin knew that if selection of particular kinds of individuals of a population takes place the next generation is affected. If the taller men of a community are selected _the average_ of their offspring will be taller than the average of the former population. If selection for tallness again takes place, still taller men will _on the average_ arise. If, amongst these, selection again makes a choice the process would, he thought, continue (fig. 79).
[Ill.u.s.tration: FIG. 78. A race of corn reared under different conditions.]
We now recognize that this statement contains an important truth, but we have found that it contains only a part of the truth. Any one who repeats for himself this kind of selection experiment will find that while his average cla.s.s will often change in the direction of his selection, the process slows down as a rule rather suddenly (fig. 80). He finds, moreover, that the limits of variability are not necessarily transcended as the process continues even although the average may for a while be increased.
More tall men may be produced by selection of this kind, but the tallest men are not necessarily any taller than the tallest in the original population.
[Ill.u.s.tration: FIG. 79. Curves showing how (hypothetically) selection might be supposed to bring about progress in direction of selection. (After Goldschmidt.)]
Selection, then, has not produced anything new, but only more of certain kinds of individuals. Evolution, however, means producing more new things, not more of what already exists.
Darwin seems to have thought that the range of variation shown by the offspring of a given individual about that type of individual would be as wide as the range shown by the original population (fig. 79), but Galton's work has made it clear that this is not the case in a general or mixed population. If the offspring of individuals continued to show, as Darwin seems to have thought, as wide a range on each side of their parents' size, so to speak, as did the original population, then it would follow that selection could slide successive generations along in the direction of selection.
[Ill.u.s.tration: FIG. 80. Diagram ill.u.s.trating the results of selection for extra bristles in D. ampelophila. Selection at first produces decided effects which soon slow down and then cease. (MacDowell.)]
Darwin himself was extraordinarily careful, however, in the statements he made in this connection and it is rather by implication than by actual reference that one can ascribe this meaning to his views. His contemporaries and many of his followers, however, appear to have accepted this _sliding scale_ interpretation as the cardinal doctrine of evolution.
If this is doubted or my statement is challenged then one must explain why de Vries' mutation theory met with so little enthusiasm amongst the older group of zoologists and botanists; and one must explain why Johannsen's splendid work met with such bitter opposition from the English school--the biometricians--who amongst the post-Darwinian school are a.s.sumed to be the lineal descendants of Darwin.
And in this connection we should not forget that just this sort of process was supposed to take place in the inheritance of use and disuse. What is gained in one generation forms the basis for further gains in the next generation. Now, Darwin not only believed that acquired characters are inherited but turned more and more to this explanation in his later writings. Let us, however, not make too much of the matter; for it is much less important to find out whether Darwin's ideas were vague, than it is to make sure that our own ideas are clear.
If I have made several statements here that appear dogmatic let me now attempt to justify them, or at least give the evidence which seems to me to make them probable.
The work of the Danish botanist, Johannsen, has given us the most carefully a.n.a.lyzed case of selection that has ever been obtained. There are, moreover, special reasons why the material that he used is better suited to give definite information than any other so far studied. Johannsen worked with the common bean, weighing the seeds or else measuring them. These beans if taken from many plants at random give the typical curve of probability (fig. 74). The plant multiplies by self-fertilization. Taking advantage of this fact Johannsen kept the seeds of each plant separate from the others, and raised from them a new generation. When curves were made from these new groups it was found that some of them had different modes from that of the original general population (fig. 81 A-E, bottom group).
They are shown in the upper groups (A, B, C, D, E). But do not understand me to say that the offspring of each bean gave a different mode.
[Ill.u.s.tration: FIG. 81. Pure lines of beans. The lower figure gives the general population, the other figures give the pure lines within the population. (After Johannsen.)]
On the contrary, some of the lines would be the same.
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