ZOOLOGICAL SCIENCE 195 



form an entire body from a portion of the whole. 8 Whether 

 such an organization is visible or not, something of the sort 

 must be present in every egg. Otherwise there can be no 

 explanation of the phenomena of heredity, which can 

 satisfy the demands of science. The embryologist is of 

 necessity a preformationist, but not in the older sense of the 

 word. 9 



In problems of this nature, satisfactory analysis can only 

 be based upon experiments which subject the organism to 

 new and controlled conditions. No observation of normal 

 development, no matter how extensive, will go so far toward 

 answering the question whether at the two cell stage the right 

 and left portions of the animal are irrevocably distributed 

 to right and left cells as will the simple experiment of sepa- 

 rating these two cells and seeing what happens. In a simple 

 way, this illustrates the whole point in the advancement of 

 zoology by means of experimentation. 



In addition to this study of embryonic stages, there is 

 another method of attacking the developmental problem, 

 which may be illustrated as follows: If one deals out the 

 hands in a game of cards and then examines the cards in 

 each hand, he can, if he knows the nature of the dealing, 

 infer the manner in which the cards were arranged in the 

 pack before the dealing began. One must assume an ar- 

 rangement within the pack, which bears a causal relation to 

 the hands or he must assume the miraculous origin of the 

 arrangement which appears as a result of the dealing. The 

 inference that such an organization exists within the pack 

 before the deal begins is of the same sort as that which the 

 chemist makes in postulating atoms and molecules which 

 have never been seen. When one studies the inheritance of 

 qualities appearing in an adult animal, it is like examining 



* Conklin, E. G., "Heredity and Environment in the Development of Men." 

 Wilson, E. B., "The Problem of Development," Science, Feb. 24, 1905. 



9 The case is analogous with that of the physicist and chemist who postulate 

 invisible molecules and atoms, as a basis for the visible phenomena. 



