XIIL j Crifirisms mt "Cftf Engirt 0f Speri**. 



Medusa; if the vermiform Trematode 'nurse* can develop within 

 itself the very unlike Cercaria, it will not appear impossible that tha 

 egg, or ciliated embryo, of a sponge, for once, under special conditions, 

 might become a hydroid polype, or the embryo of a Medusa, an 

 Echinoderm." 



It is obvious, from these extracts, that Professor Kol- 

 liker's hypothesis is based upon the supposed existence 

 of a close analogy between the phsenomena of Agamo- 

 genesis and the production of new species from pre- 

 existing ones. But is the analogy a real one? We 

 think that it is not, and, by the hypothesis, cannot be. 



For what are the phenomena of Agamogenesis, stated 

 generally? An impregnated egg develops into an 

 asexual form, A ; this gives rise, asexually, to a second 

 form or forms, B, more or less different from A. B may 

 multiply asexually again ; in the simpler cases, however, 

 it does not, but, acquiring sexual characters, produces 

 impregnated eggs from whence A once more arises. 



No case of Agamogenesis is known in which, when A 

 differs widely from B, it is itself capable of sexual 

 propagation. No case whatever is known in which the 

 progeny of B, by sexual generation, is other than a 

 reproduction of A. 



But if this be a true statement of the nature of the 

 process of Agamogenesis, how can it enable us to com- 

 prehend the production of new species from already 

 existing ones ? Let us suppose Hyaenas to have pre- 

 ceded Dogs, and to have produced the latter in this 

 rvvay. Then the Hyaena will represent A. and the Dog, 

 B. The first difficulty that presents itself is that the 

 Hysena must be asexual, or the process will be wholly 

 without analogy in the world of Agamogenesis. But 

 passing over this difficulty, and supposing a male and 

 female Dog to be produced at the same time from the 

 Hyeena stock, the progeny of the pair, if the analogy 



