42 DISTINCTION BETWEEN SPECIES AND VARIETIES. 



many differences of this kind, in the markings of the surface ; but 

 others of a still more important character are often met with. 

 Thus, there are some species, in vvhich the females are destitute 

 of wings, as is the case with the common Glow-worm ; and 

 others, in which her form is quite different from that of the 

 adult male (although the young of the two sexes correspond), 

 of which we have an example in the Cochineal insect. But even 

 these differences are surpassed by those which are met with in 

 other classes. Thus among the lower Crustacea, there are seve- 

 ral species, of which only females have been hitherto discovered ; 

 whence, it is probable, that the males possess an entirely different 

 form, and have not improbably been described as members of 

 some other group. In the curious Lerncea (Fig. 7-) the male is 

 much smaller than the female, and differs so much in form, that 

 its true character was long unknown. 



20. From these facts we are led to perceive, how much 

 knowledge is required by the Naturalist, before he determines 

 upon the arrangement of the different forms of animal-life, which 

 he has collected, into distinct species, the first step in classifica- 

 tion. It is not sufficient that, on comparing together two 

 animals which bear a strong general resemblance to one another, 

 he finds some obvious point of difference. He must ascertain 

 whether this difference invariably manifests itself, in the races to 

 which these animals respectively belong ; or whether it is a mere 

 individual peculiarity. Thus, Men with six fingers and six toes 

 are occasionally to be met with ; but we should not on this 

 account rank them as constituting a species distinct from ordi- 

 nary men, since both the parents and offspring, the brothers and 

 sisters, of such persons, are very commonly found to have only 

 the usual number. Yet, a variation of no greater amount among 

 many of the lower tribes, especially Insects, would be a suffi- 

 cient distinction, not only between different species, but between 

 different genera, or even families.* But, even supposing that 

 such variations do appear constant, it is further necessary to 

 ascertain, whether they have always been so, or whether they 



* Thus the great order, Coleoptera, or Beetles, is divided into sub-orders, 

 according to the number of joints in the foot. 



