CONCLUSION 327 



to adapt itself in a constant manner to its surroundings. 

 Nothing could better illustrate the effects of this distance- 

 action than such experiments as M. L. Pezard has performed 

 upon Birds, showing that not only their external appearance 

 but their psychology may be changed — by castration, for 

 instance, or genital grafts. 1 These operations profoundly 

 modified the development of the cock's plumage, and even 

 incited the hen to adopt his crow — a purely psychological 

 effect. It is not surprising, then, that these modifications 

 should react upon the structure of the reproductive cells them- 

 selves in order to become functional, which from the time of the 

 repeated segmentation whereby they are able to reconstitute 

 an organism similar to the one from which they came, must 

 recapitulate, in inverse order, the stages through which the 

 latter has passed in order to reach its final form. This is what 

 constitutes heredity. It perpetuates the individual in his 

 progeny. But the substances which accumulate within the 

 individual not only modify him ; unfortunately they encumber 

 him, and he ends by succumbing to the burden, after passing 

 through the phase of gradual decay which we call old age. 



The regeneration through the reproductive cells of the 

 successive characters of the ancestral organism from which 

 they came, led Etienne Geoffroy Saint-Hilaire to conceive the 

 embryogeny of living forms, both animal and vegetable, as 

 consisting in a rapid epitome of their descent. The 

 increasing rapidit}' with which the ancestral characters succeed 

 one another in an embryo, unequal though it be for different 

 organs, ensures that these characters are finally telescoped one 

 into another, so to speak, while, at the same time, those which 

 evolve most rapidly by a sort of inter-organic struggle for 

 existence take the place and absorb the nourishment of those 

 that develop more slowly. To this acceleration in the succession 

 of embryogenetic phenomena, resulting from the definitive 

 modification of the adult form, we have given the name of 

 tachygenesis. Thanks to its influence, heredity becomes, by a 

 sort of paradox, a modifying instead of a conservative force. 

 The importance of tachygenesis as a cause of organic trans- 

 formation cannot be over-estimated. We have seen how it 

 produced the Vertebrate type. But tachygenesis is itself a 



1 Le conditionnement physiologique des caracteres sexuels secondaires chez 

 les Oiseaux. These de Paris (Sciences), 1918. 



