THE NATURE OF REPRODUCTION. 571 



popular experience it appears a natural necessity. In reality it is one 

 of the most remarkable phenomena connected with the generative 

 process; and it indicates an unbroken connection of physiological acts, 

 extending through the lives of many different individuals. Thus we 

 know that the progeny of a fox will always be foxes ; and that if we 

 sow oats, it will be a crop of oats that is produced in consequence. 

 Generation, accordingly, not only gives rise to new animals or plants, 

 but it serves to continue indefinitely the existence of the species, with 

 its characteristic marks and qualities. 



Our idea, therefore, of a species includes two different elements, one 

 of which is anatomical, the other physiological. Its anatomical char- 

 acter is the similarity of form, size, and structure between the individ- 

 uals belonging to it, which we recognize at a glance ; its physiological 

 character is the fact, established by experience, that it will reproduce 

 itself, and thus remain distinct through an indefinite series of succes- 

 sive generations. 



It is not possible to say that the anatomical characters of a species 

 have been absolutely the same throughout all previous time, or that 

 they will remain so without limit in future. The fossil remains of 

 animals and plants, differing from those now in existence, show that 

 species are not persistent and invariable through very long periods ; 

 and that they may either become so modified as to present a different 

 appearance, or may entirely come to an end, like the extinct mastodons 

 and horses of the United States, and be replaced by others from a dif- 

 ferent locality. But in whatever way we may explain the geological 

 succession of different species, it is certain that at any one time their 

 essential characters are those above described ; and that each species, 

 by the process of generative reproduction, remains distinct from the 

 others which are contemporary with it. 



But the production of young animals, similar to their parents, although 

 the final result of the generative process, is never immediate. The 

 young progeny is at first different from its parents, and only comes 

 to resemble them through a series of changes, often very remarkable in 

 kind. The embryo of a vertebrate animal, though incomplete in struc- 

 ture, presents some analogy of form with the adult. But in many 

 invertebrates, the young, even when hatched and capable of active 

 locomotion, are so different from their parents that they would never 

 be referred to the same species, unless their identity were demonstrated 

 by subsequent development. The young mosquito is a wingless crea- 

 ture living beneath the surface of stagnant pools; and the eggs of the 

 butterfly, when hatched, give birth not to butterflies but to caterpillars. 

 The caterpillars, however, are not creatures of another species, but 

 young butterflies ; and they become similar to their parents after cer- 

 tain changes, which take place at definite periods of their development. 



The reproduction of form, therefore, which marks a species, is accom- 

 plished through a series of changes in regular order ; and this series, 

 taken together, may be represented by a circuit, which starts from the 



