W. K. BROOKS ON THE GENUS SALPA. 159 



fauna, or what these starting-points were like. Our inability to describe 

 or picture these ancestral forms is no reason for doubting their reality, 

 for in biology the weight and certainty of a deduction are often indepen- 

 dent of its definiteness. We may, for example, feel sure that the cetacea 

 are descended from terrestrial animals and yet find it impossible to 

 picture their ancestor, or even to decide whether their ancestral lines 

 converge into one stem before or after the pelagic habit was acquired. 



We may in the same way feel sure, even in the absence of sufficient 

 evidence to trace their direct paths, that all the great groups of metazoa 

 ran back to minute pelagic ancestors, and we must, therefore, include in 

 the primitive pelagic fauna a great, but indefinite, number of distinct 

 and somewhat widely separated ancestral forms, and together with 

 them, no doubt, an equal or greater number of somewhat similar forms 

 which have been exterminated and have left no descendants. In these 

 extinct forms we should, if we could study them, find the connecting 

 links between divergent groups, and we would thus be able to complete 

 the genealogical tree of the metazoa by bringing together the great 

 divergent branches of the metazoan stem whose primary relationships 

 now seem beyond discovery. 



In addition to the primitive pelagic animals which are known to us 

 only by the traces of their former existence which they have left in the 

 structure and habits of. modern larvae and embryos, there are a few 

 modern pelagic adult animals which show by their minute size and 

 simple structure and by their systematic affinities that they are primi- 

 tive pelagic animals, owing their structure to purely pelagic influences. 



Appendicularia is a good example of this class, and I believe that the 

 copepods are the most important group of the primitively pelagic metazoa. 



The Origin of the Crustacea. 



The view that the copepods are degenerated descendants from 

 Crustacea like the phyllopods, and that the Crustacea were evolved on the 

 bottom, and that the pelagic habit of the copepods is 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 explanation of 

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

 an influence which tends to metamerism at the present day. 



