40 



DISTINCTION BETWEEN SPECIES AND VARIETIES. 



striking variations in many animals, not only in the size of the 

 body, but in its form and structure, at different periods of life ; 

 and also, not un frequently, a great want of resemblance between 

 the two sexes. In some instances, there is a positive metamor- 

 phosis, or change of form, between the early age of the animal, 

 and its adult or complete state ; this is the case in the Frog 

 tribe, in Insects, and in many of the lower classes. No one 

 who might be unacquainted with the history of these changes 

 would hesitate in regarding the Caterpillar as belonging, not 

 only to a species, but even to a class, distinct from the Insect ; or 

 in separating the Tadpole from the Frog ; and yet, by due 

 attention to the history of these animals, we come to know that 

 one is transformed into another widely different. 



18. Now, although Zoologists are tolerably well acquainted 

 with such changes of this description as take place among the 

 higher classes, yet their knowledge is still by no means so perfect 

 of the metamorphoses of the lower. Thus it is not very many 

 years since satisfactory evidence was obtained, that the Zoea, a 

 little crustaceous animal, long regarded as forming a genus by 

 itself, is nothing else than the young of 

 the common Crab; and that an animal 

 very closely resembling it undergoes a 

 still more extraordinary metamorphosis, 

 in order to assume its ultimate form of 

 the Barnacle. But even where no such 

 well-marked changes occur during the 

 period of growth, there are often varia- 

 tions which would be held to distin- 

 guish species, if we were not aware that 

 the two forms are really the same, in 

 different states. Thus, in almost all 

 Birds, the plumage of the young male is 

 different from that of the adult ; some- 

 times resembling that of the adult 

 female ; but often being different from 

 that of either parent. Even here, then, we should be in danger 

 of failing into error, by separating as distinct species what are 



Fi EARLY FORM or THE 



CRAB. 



