BIOLOGICAL INVESTIGATIONS. I 33 



Structural details have been worked out and found to corre- 

 spond to those of the ontogeny, and several authors now de- 

 scribe these different stages of the phylogeny by the use of 

 compound terms derived from phylum and the name of the 

 corresponding stage in the ontogeny. They are in this way 

 enabled to designate the primitive types of the phylum as 

 phylembryonic, the nearer ancestors as phyloneanic, and the 

 full-blown acme as phylephebic. 



But, as stated above, this does not finish the history of 

 correlations. Taking up next the retrogressive phase of the 

 ontogeny, similar methods have shown that the retrogressive 

 stages in the evolution of the phylum correspond to those that 

 occur in old age or the gerontic stage, and by a similar termi- 

 nology we have been able to describe the retrogressive stages 

 of the phylum as being phylogerontic. 



There are, however, very essential differences, in the beha- 

 vior of the correspondences of the ontogeny and phylogeny, 

 between the progressive and retrogressive phases. It will at 

 once strike most thinkers that progressive changes in the on- 

 togeny take place, as a rule, before the reproductive period, and 

 have consequently a more direct influence upon the phylogeny. 

 Their recapitulation in the ontogeny may therefore be caused 

 by heredity, while the recapitulation of the retrogressive char- 

 acters, which occur in the phylogeny after this period is passed, 

 may perhaps not be caused by heredity, but may be due to the 

 necessarily similar character of degenerative changes in similar 

 organisms having a common origin. 



Biologists have not as yet investigated the limits of the 

 reproductive period in the different stages of development of 

 animals, and it is therefore not practicable to solve this ques- 

 tion. Who knows, for example, what the limit of the reproduc- 

 tive stage is in a clam or a lobster or any of the invertebrates } 

 That these have senile changes is easily seen, but whether the 

 gerontic stage begins before or after the power of reproduction 

 is lost, if it ever is in the ontogeny of some of these animals, 

 no one seems to know. Certainly in many invertebrates, and 

 in as high a type as Mammalia, senile degeneration sets in long 

 before the reproductive power has disappeared. 



