EMBRYOLOGICAL CRITERION OF HOMOLOGY. 123 



without further knowledge of its physiological aspect. I be- 

 lieve that until this knowledge is forthcoming the embryological 

 criterion of homology must remain of relatively small value, 

 and be held in subordination to the anatomical. 



The all-important need of embryology at the present day 

 is the study of embryonic physiology. In this direction ex- 

 perimental embryology has opened the way to an apparently 

 unlimited field of research ; and there is reason to hope that 

 here, as in the physical sciences, the study of phenomena under 

 artificially modified and simplified conditions, will give us a 

 deeper insight into the more complex conditions existing in 

 nature. The greatest fault of embryology has been the ten- 

 dency to explain any and every operation of development as 

 merely the result of "inheritance," overlooking the vital point 

 that every such operation must have some physiological mean- 

 ing for the individual development, hard though it may be to 

 discover. We have still but the most rudimentary notion of 

 what the physiological conditions of development are, and how 

 they operate, but they must be thoroughly investigated before 

 the reform of embryological morphology can be carried out, 

 and here experimental embryology and physiological morphology 

 must lead the way. But, on the other hand, it is no less essen- 

 tial not to neglect the study of phenomena where nature is the 

 experimenter. While it is true that the normal operations 

 of development are essentially physiological problems, we 

 must, nevertheless, not lose sight of the cardinal fact that 

 the organization of the idioplasm, which is at the bottom 

 of every such operation, is an inJieritance from the past. 

 The idioplasm of every species has arisen through the modi- 

 fication of a preexisting idioplasm, and every response that it 

 gives to stimulus is an expression of its past history. Hence, 

 we need not despair of ultimate success in the attempt to de- 

 cipher the meaning of the embryological record, and to find in 

 ontogeny a real criterion of homology ; and it is here that we 

 find encouragement, were any needed, not to relax our efforts 

 to investigate the normal phenomena of comparative embry- 

 ology on the largest scale, and down to the minutest detail. I 

 do not belong to those who, impressed by the rich fruits and 



