SALPA IN RELATION TO EVOLUTION OF LIFE. 151 



secondary, is so generally accepted that it is hardly worth while 

 to advance a different view in this place where there is no room 

 for its exhaustive treatment. 



The consideration which seems to have the greatest weight with 

 morphologists is the supposed necessity of a phylogenetic explana- 

 tion of metamerism, but a little reflection will show the persistent 

 existence of an influence which tends to metamerism at the 

 present day. 



For this influence, which is shown by such phenomena as the 

 inheritance by the child of polydactylous feet from the polydacty- 

 lous hands of the parent, or the development of wing- feathers on 

 the toes of fantail pigeons ; the influence which has carried the 

 feet of the horse family along the same line of evolution with the 

 hands, I have, in another place, proposed the term ontogenetic 

 inheritance. Among the arthropods, examples of this sort of 

 modern metamerization are very common, both as normal features 

 of their structure, in the movable body-rings of the ocular and 

 antennary somites of stomatopods, for example, and as monstrosi- 

 ties, as in the twelve-legged coleoptera. 



I believe that a thorough study of this most interesting and 

 instructive class of facts will convince any one that there is no 

 philosophical necessity for assuming that the primitive crustacean 

 had a highly metamerized body like that of a phyllopod, and that 

 all the common features in the structure of arthropods may have 

 been derived from a common ancestor as simple as a nauplius. 



The analogy between the parapodia of annelids and the limbs 

 of Crustacea has been held to prove that the primitive crustacean 

 limb was not a rowing organ fitted for a pelagic life, like the limbs 

 of the nauplius and the copepod, but flat and leaf-like and adapted 

 for movement over the bottom. 



It is hardly possible, however, to believe that the arthropods 

 have been derived from the higher polychsetous annelids, and as 

 the simpler and more primitive annelids have no parapodia, the 

 resemblance, which is not actually very noteworthy, can be noth- 

 ing more than an analogy. 



There are plenty of degenerated copepods, and we have in their 

 structure abundant proof of the degeneracy, and an adequate 

 explanation of it in their parasitic habits, but they are degenerated 



