History of the Theory of Heredity. 33 



cell, and each when placed under the microscope wil] 

 soon be seen to pass through almost exactly tlie same 

 changes, giving rise by division to a spherical layer of 

 cells. Yet if these three eggs are placed together in 

 a tumbler of water and exposed to identical conditions, 

 one may at last become a star-fish, another a crustacean, 

 and another a vertebrate. Similar things under similar 

 conditions cannot give rise to widely different results, 

 and there seems no escape from the conclusion that 

 these three eggs are not similar, or even essentially 

 alike, but that one of them is a potential star-fish, 

 another a potential crustacean, and a third a potential 

 vej'tebrate. That there is in each of them a something 

 which separates it very Avidely from the other two, and 

 determines its future history. 



The hypothesis of epigenesis j^roves, then, on careful 

 analysis to be as unsatisfactory as the speculations of 

 Bonnet and Buiion, and we must acknowledge that we 

 are as yet unable to picture to ourselves the hidden sig- 

 nificance of the phenomena of individual development, 

 without returning to some modification of the old evolu- 

 tion hypothesis.. 



The attem])t to escape this necessity, and to hold fast 

 to the hypothesis of epigenesis, has given rise within 

 recent years to much ingenious speculation, and an ex- 

 amination of some of the published papers will help, 

 rather than retard, our argument. 



Among these, one of the most ingenious and sug- 

 gestive is Haeckel's paper, ^^ Ueber die AYellenzengung 

 der Lebenstheilchen oder die Perigenesis der Plasti- 

 dule." The following extract ('^ Gesammelte Populare 

 Vortrage," II., pp. 66-72) will, I hope, give a suffi- 

 ciently clear statement of his views: 



**In order to penetrate still farther into tlie mechan- 



