ALTERNATE AND EQUIVOCAL REPRODUCTION. 159 



mission of these characteristics, from one generation to the 

 next, is justly considered as one of the great laws of the 

 Animal and Vegetable Kingdoms. It is, indeed, one of the 

 points on which the definition of species is generally founded. 

 We would, however, unhesitatingly adopt the new definition 

 of Dr. S, G. Morton, who defines species to be " primordial 

 organic forms." 



335. But it does not follow that animals must resemble 

 their parents in every condition, and at every epoch of their 

 existence. On the contrary, as we have seen, this resem- 

 blance is very faint, in most species, at birth ; and some, 

 such as the caterpillar and the tadpole, undergo com- 

 plete metamorphoses before attaining their final shape as the 

 butterfly and frog. Nevertheless, we do not hesitate to refer 

 the tadpole and the frog to the same species ; and so with the 

 caterpillar and the butterfly ; because we know that there is the 

 same individual observed in different stages of development. 



336. There is, also, another series of cases, in which the 

 offspring not only do not resemble the parent at birth, but, 

 moreover, remain different during their whole life, so that 

 their relationship is not apparent until a succeeding genera- 

 tion. The son does not resemble the father, but the grand- 

 father ; and in some cases the resemblance reappears only 

 at the fourth or fifth generation, and even later. This sin- 

 gular mode of reproduction has received the name of alter- 

 nate generation. The phenomena attending it have been 

 of late the object of numerous scientific researches, which 

 are the more deserving of our attention, as they furnish a 

 solution to several problems alike interesting in a zoological 

 and in a philosophical point of view. 



337. Alternate generation was first observed among tl e 

 Salpae. These are marine mollusks, without shells, belong- 

 ing to the family Tunicata. They are distinguished by ihu 

 curious peculiarity of being united together in considerable 

 numbers so as to form long chains, which float in the sea. 



