Conklin.l *^ [May 1, 



Discussion of the Factors of Organic Evolution from the Embryological 



Standpoint. 



By Frof E. G. Conklin. 



{Read before the American Philosophical Society, May 1, 1SD6.) 



Up to the beginning of this decade embryology was largely domi- 

 nated by the phj^logeny idea. Individual development was generallj' 

 studied, as the paleontologist studies his fossils, with a view to 

 deciphering the evolutionary record in the various stages. It is now 

 generally recognized, however, that embryology is but little fitted for 

 the service into which it was so long forced, viz., the determining of 

 phylogenies. The only safe guide in this matter is comparative anatomy 

 of both living and extinct forms. On the other hand, our knowledge 

 of the mechanics of evolution must always depend in large part upon 

 the study of individual development. More than any other discipline, 

 embryology holds the keys to the method of evolution. If ontogeny is 

 not a true recapitulation it is, at least, a true type of evolution, and the 

 study of the causes of development will go far to determine the factors 

 of phylogeny. 



The causes and methods of evolution are intimately bound up with 

 those general phenomena of life such as assimilation, growth, differen- 

 tiation, metabolism, inheritance, and variation ; and the evolution 

 problem can never be solved except through a study of these general 

 phenomena of life itself. Our great need at present is not to know 

 more of the course of evolution, but to discover, if possible, the 

 causes of growth, differentiation, repetition, and variation. All these 

 general phenomena are most beautifully illustrated in the develop- 

 ment of individual organisms, and because they are fundamental 

 to any theory of evolution I shall dwell upon them rather than upon 

 the evidences for the Lamarckian or the Darwinian factors. 



I call your attention very brieflj^ to the following propositions : 1. 

 Development, and consequently evolution, is the result of the interac- 

 tion of extrinsic and intrinsic causes. 2. Intrinsic causes are dependent 

 upon protoplasmic structvire. 3. Inherited characters must be prede- 

 termined in the structure of the germinal protoplasm. 4. Germinal, 

 as compared with somatic, protoplasm is relatively stable and contin- 

 uous, but not absolutely so as maintained by Weismann ; therefore, 

 extrinsic causes may modify both germinal and somatic protoplasm. 

 5. It is extremely difficult to determine whether or not extrinsic factors 

 have modified the structure of the germinal protoplasm. This is illus- 

 trated by some of the evidences advanced for the inherited efi'ects of 

 (1) diminished nutrition, (2) changes in environment, (8) use and 

 disuse. 6. Experiment alone can furnish the crucial test of these 

 Lamarckian factors. 



