IXTRODUCTIOX 5 



it has no vertebrae; on the other hand, it has a heart of an am- 

 phibian rather than of a fish type. 



Some of these considerations may be represented graphically 

 as follows : let us take a species D that has an ontogeny A, B, C, D, 

 and suppose that this species evolves successively into species 

 E, F, G, H, etc. When evolution has progressed a step, to E, 

 the characters of the species established develop directly from 

 the ovum, and are therefore, in some way, involved in the com- 

 position of the latter. All of the stages of the ontogeny leading 

 up to E are modified, and we can indicate this in the ontogeny 



1. A B C D of E as in line 2; similarly, when evolu- 



2. A^ B^ C^ D^ E tion has progressed to species F, seeing 



3. A^ B^ C^ D^ E^ F that the characters of F now develop 



4. A^ B^ C^ D^ E^ F^ G directly from the ovum, all the onto- 



5. A'' B'' C^ D" E^ F^ G^ H genetic stages leading up to F are modi- 

 fied, line 3. And so on for each successive advance in evolution, 

 lines 4 and 5. It will also be noticed that the terminal stage D of 

 species 1, becomes a successively earlier ontogenetic stage of species 

 2, 3, 4, 5, etc., and moreover it does not recur in its pure form, 

 but in the form D^ in species 2, D^ in species 3, etc. Now if the 

 last five stages of the ontogeny of species 5 be examined, viz., 

 D"*, E^, F^, G^, H, it will be seen that they repeat the ph3'logeny 

 of the adult stages D, E, F, G, H, but in a modified form. 



This is in fact what the diagram shows; but it is an essential 

 defect of the diagram that it is incapable of showing the character 

 of the modifications of the ancestral conditions. Not only is each 

 stage of the ancestral ontogenies modified with each phylogenetic 

 advance, but the elements of organization of the ancestral stages 

 are also dispersed so that no ancestral stage hangs together as a 

 unit. The embryonic stages show as much proportional modi- 

 fication in the course of evolution as the adult, but this is not 

 so obvious owing to the simpler and more generalized character 

 of the embryonic stages. 



The recapitulation theory as outlined above is obviously a 

 corollary of the theory of organic descent; it was in fact developed 

 in essentially its present form, soon after the publication of the 

 "Origin of Species," by Fritz Miiller and Ernst Haeckel. But 

 the data on which it was based were known to the earlier embry- 

 ologists; and ^leckel, for instance, insisted very strongly on the 

 resemblance between the ontogenetic and the taxonomic series 



