STRUCTURE AND DEVELOPMENT 71 



intricate series of transformations. When life-histories 

 were more closely compared, the meaning of the resem- 

 blances between early stages of diverse adult organisms 

 was read by the same method which in comparative 

 anatomy finds that consanguinity is expressed by re- 

 semblance. The great law of recapitulation, stated in 

 one form by Von Baer and more definitely by Haeckel 

 in the terms employed in the foregoing sections, was for 

 a time too freely used and too rigidly applied by natu- 

 ralists whose enthusiasm clouded their judgment. A 

 strong reaction set in during the latter part of the nine- 

 teenth century, when attention was directed to the 

 anachronisms of the embryonic record and to the 

 alterations that are the results of larval or embryonic 

 adaptation as short cuts in development. Neverthe- 

 less, it is not seriously questioned, I believe, that the 

 main facts of a single life-history owe their nature to 

 the past evolution of the species to which a given animal 

 belongs. 



Nowadays the problems in this well-organized de- 

 partment are concerned not only with more accurate 

 accounts of the development of animals, but also with 

 the mechanics of development, with the relative value of 

 external and internal influences, and above all with the 

 physical basis of inheritance. It is clear that the 

 factors that direct the development of a wood frog's 

 egg so that it becomes a wood-frog and not a tree-toad 

 must lie in the egg itself, as derivatives from the two 

 parent organisms. Weismann and his followers have 

 proved that a peculiar substance in the nuclei of the 

 egg and its daughter-products contains the essential 

 factors of development, whatever these may be. Ex- 



