CASES SIMULATING ALTERNATION OF GENERATIONS. 22o 



common subject of remark, as well as the tendency of cer 

 tain hereditary diseases, as gout, to reappear in the grand- 

 son, rather than in the immediate issue. 



4. By some authors the phenomena of Metamorphosis, 

 as they occur in Insects, Crustaceans, Batrachians, &c., have 

 been confounded with those of alternation. Such meta- 

 morphosis is described by Dr. A. Thomson as a breach 

 of continuity in the progress of embryonic development, 

 " marked by some change in the mode of life, or some dif- 

 ference in the structure of the individual"* a definition 

 which might perhaps be stretched to include cases of 

 alternation. In almost all cases of metamorphosis there is 

 a separation of what was originally a single body into more 

 parts than one, inasmuch as there is always something 

 cast off either, as in Insects and Crustaceans there is 

 ecdysis or casting of the skin (what Huxley terms concentric 

 fissionf") or, as in the Ascidians and some Batrachians, 

 the casting off of a member, as the tail. The difference in 

 so far is perhaps one rather of accident than of essence, and 

 seems to consist in this, that in the metamorphosis none of 

 the parts but one is capable of continuing its existence, so 

 that, disregarding all the others, we at once identify the 

 living segment with the animal which has been the subject 

 of the metamorphosis ; whereas in well-marked cases of 

 alternation, the vitality being more equally divided, we 

 have in the swarm of resulting zooids, so many candidates 

 for identity with the pre-existing structure, that we in- 

 voluntarily cancel their claims as destructive of each other's 

 validity, and regard them all alike as the progeny of the 

 pre-existing structure, and not as a continuation of its very 

 self. That there is not merely this community in theory, 

 but that in actual nature the two cases do tend to merge 

 into each other has already been pointed out in speaking of 



* Cycl. Anat. and Phys. Sup. [131] (ovum). 



f Lect. at Royal Inst. Ann. Nat. Hist., 2d Ser., IX., 505. 



