BIOLOGICAL INVESTIGATIONS. 



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ble into five periods — the embryonic or earliest usually encysted 

 or ovarian stages, the nepionic or baby stage, the neanic or 

 adolescent, the ephebic or adult, and the gerontic or senile 

 stage. These pass insensibly into each other and have two 

 phases — the progressive, leading to the full-blown adult, and 

 the retrogressive, leading by senile degeneration to the final 

 destruction of organic continuity in death. 



The second class of movements (2) take place on a larger 

 scale, and have been designated Phylogeny. This is the life 

 history of the stock, but is more generally conceived of as 

 the evolutionary history of any series of genetically connected 

 forms. 



If the ontogeny be granted as possessing capacity for change 

 and the power of reproduction, the phylogeny becomes neces- 

 sarily a function or product of the ontogeny. This is so obvi- 

 ous that it would need no explanation but for the fact that at 

 present the appearance of variations in the young is supposed 

 by some to indicate fundamental inherent tendencies of organ- 

 isms to depart from the parent stock. 



The mass of evidence and all the obvious phenomena, how- 

 ever, show the identity of reproduction and heredity ; they are 

 both necessarily the production of like by like. The simplest 

 forms of reproduction, the division of the unicellular Protozoa, 

 the formation of colonies among Protozoa, and all the asexual 

 modes of reproduction have this character in the Metazoa. 

 The sexual modes of reproduction do not alter these funda- 

 mental relations, except in so far as the spermatozoa graft dif- 

 ferent elements upon the ovum, which influence the subsequent 

 characters of the organism. The phenomena of alternation 

 of generations in Hydrozoa and the hyper-metamorphoses of 

 insects cannot be cited against the old views of heredity and 

 reproduction, since the cycle of the ontogeny in these invari- 

 ably swings around to the same form, however great the devia- 

 tion of the intermediate generations or stages of development. 



Variations in any given forms are, as a rule, at first infinitely 

 slight as compared with the vast mass of recapitulated charac- 

 ters. They are necessarily grafts upon stock structures until 

 they become incorporated with them, and then are subject to 



