150 W. K. BROOKS. 



forth, although we are quite unable to say how many independent 

 starting-points these various metozoic lines had in the primitive 

 pelagic 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 independent 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 suffi- 

 cient 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 exter- 

 minated 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 metazoic stem whose primary relation- 

 ships 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 larvse 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 primitive 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 



