SOME GENERAL CONCLUSIONS 463 



of generations, they may in time bring about an appreciable lasting 

 change in the whole system of such a kind that the characteristics 

 produced by them will become hereditary. And if the cells which 

 give rise to gametes are integral parts of the organism, such a 

 change must sooner or later affect them as well as other parts. It 

 is quite impossible to discuss at this time the great mass of evidence 

 for and against the inheritance of these so-called acquired charac- 

 ters. In general, biologists have been slow to admit the possibility 

 of such inheritance, largely because it conflicts with the Weisman- 

 nian theory, but if we admit that the gametes are integral parts of 

 the organism, there is no theoretical difficulty in the way of such 

 inheritance. Whatever the theoretical possibilities may be, it is in 

 my opinion quite impossible to account for the course of evolution 

 and particularly for many so-called adaptations in organisms with- 

 out the inheritance of such acquired characters, but since thousands 

 or tens of thousands of generations may be necessary in many cases 

 for inheritance of this kind to become appreciable, it is not strange 

 that experimental evidence upon this point is still conflicting. 



The morphological parallelism between the course of individual 

 development and the course of evolution have long been familiar 

 to biologists and have been the subject of much discussion and 

 speculation. While departures from this parallelism are numerous 

 and often conspicuous, nevertheless the so-called biogenetic law 

 that embryology repeats phylogeny, i.e., the development of the 

 individual repeats evolutionary history, still remains a striking 

 biological fact. Moreover, a physiological parallelism seems to 

 exist to some extent. In the individual we see advancing diversity 

 and specialization of function, apparently associated with increas- 

 ing stability of the structural substratum, and in evolution a similar 

 series of changes. The question at once arises : Can we not find a 

 clue in individual development to certain factors concerned in 

 evolution ? 



In earlier chapters I have attempted to show that individual 

 development and senescence are associated with the increase in 

 stability of the substratum, while regression and rejuvenescence 

 involve a return to the original "undifferentiated' active proto- 

 plasmic condition. It is of course not necessary to assume that in 



