INFLUENCE OF AGE AND SEX. 41 



really the young and adult states of the same, were we not to 

 study the animal through its whole period of life. The same is 

 the case, too, in regard to Fishes ; the markings on which un- 

 dergo a similar variation ; so that it has been only lately ascer- 

 tained with certainty (the difficulty of observation being here 

 greater) that the Salmon-Parr is the young of the Salmon, and 

 not a distinct species, as it was long considered. Among Shells, 

 similar difficulties exist, the young being often very different 

 in form from the adult (as will be shown hereafter), so as only 

 to be identified with it as the same species, by comparing toge- 

 ther a number of specimens in different stages of growth. Such 

 a series is, of course, more difficult to obtain among fossils, than 

 among shells of existing races ; and it is consequently very often 

 difficult to speak with certainty, as to whether two fossil shells 

 are of the same, or of different species. In some groups, on the 

 other hand, there is such a similarity among the different shells, 

 and such a gradual passage from one form to another, that it is 

 very difficult to say, from the shells alone, whether any distinc- 

 tion of species exists at all. It is to be remembered that the 

 shells are only the external skeletons, thrown off from the 

 surface of the animals which form them ; and that it is conse- 

 quently not at all safe to judge from them alone; since differences 

 may exist in the animals, where the shells do not manifest any. 

 19. The difference of sex, too, is often marked by such dif- 

 ferences in the form and colouring of the body, and even in the 

 shape of parts which might have been expected to be alike in the 

 male and female of the same species (the antenna of insects for 

 example), as may often perplex the Naturalist. Thus, it is well 

 known, that, among Birds, the male is usually larger and more 

 vigorous than the female, his plumage gayer, and his song more 

 powerful and varied ; and where the head has any distinctive 

 crest of feathers, this is often wanting in the female. Among 

 Mammals, too, there are several species in which horns are con- 

 fined to the rnale-sex ; and in the Lion we have a familiar in- 

 stance of the difference of aspect between the male and female, 

 caused by the greater quantity and length of the hair on the 

 head and neck of the former. Among Insects, again, there are 



