De Vries’s new “mutation theory” is clearly not an alternative but a complementary theory to natural selection, the Weismannian and Mendelian theories.  Like these last, it emphasizes the importance of the congenital hereditary qualities contained in the germ plasm, though unlike the Darwinian doctrine it shows that sometimes new forms may arise by sudden leaps and not necessarily by the slow and gradual accumulation of slight modifications or fluctuations.  The mutants like any other variants must present themselves before the jury of environmental circumstances, which passes judgment upon their condition of adaptation, and they, too, must abide by the verdict that means life or death.

From what has been said of these post-Darwinian discoveries, the Lamarckian doctrine, which teaches that acquired non-congenital characters are transmitted, seems to be ruled out.  I would not lead you to believe that the matter is settled.  I would say only that the non-transmission of racial mutilations, negative breeding experiments upon mutilated rats and mice, the results of further study of supposedly transmitted immunity to poisons—­that all these have led zooelogists to render the verdict of “not proved.”  The future may bring to light positive evidence, and cases like Brown-Sequard’s guinea-pigs, and results like those of MacDougal with plants, and of Tower with beetles, may lead us to alter the opinion stated.  But as it stands now most investigators hold that there are strong general grounds for disbelief in the principle, and also that it lacks experimental proof.

* * * * *

The explanation of natural evolution given by Darwinism and the principles of Weismann, Mendel, and De Vries, still fails to solve the mystery completely, and appeal has been made to other agencies, even to teleology and to “unknown” and “unknowable” causes as well as to circumstantial factors.  A combination of Lamarckian and Darwinian factors has been proposed by Osborn, Baldwin, and Lloyd Morgan, in the theory of organic selection.  The theory of orthogenesis propounded by Naegeli and Eimer, now gaining much ground, holds that evolution takes place in direct lines of progressive modification, and is not the result of apparent chance.  Of these and similar theories, all we can say is that if they are true, they are not so well substantiated as the ones we have reviewed at greater length.

The task of experimental zooelogy is to work more extensively and deeply upon inheritance and variation, combining the methods and results of cellular biology, biometrics, and experimental breeding.  We may safely predict that great advances will be made during the next few years in analyzing the method of evolution; and that a few decades hence men will look back to the present time as a period of transition like the era of reawakened interest and renewed investigation that followed the appearance of the “Origin of Species.”  For the present, we can justly say “that evolution, so far as it is understood, is a real and natural process.”