128 REPRODUCTION. 



ance, by which their peculiar organization is indicated. 

 The transmission of these characteristics, from one gene- 

 ration 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 

 founded. 



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 of 

 them, such as the butterfly and the frog, undergo complete 

 metamorphoses, before attaining their final shape. Never- 

 theless, 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 it 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 resembles not 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 propagation has received the name of alter- 

 nate reproduction. 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 reproduction was first observed among 

 the Salpse. These are marine mollusks, without shells, be- 

 longing to the family Tunicata. They are distinguished 

 by the curious peculiarity of being united together in 

 considerable numbers, the mouth (m) being free, so as to 



