194 The American Naturalist. [March, 



each of which was the progenitor of one of these great branches 

 of the tree of life. 



We are here in a realm of the unknown, through which we 

 are forced to make our way slowly and uncertainly by aid of 

 the clues of embryology, microscopic life conditions, principles 

 of variation and development, an<J the known conditions of 

 pelagic life. We can only surmise that, as the result of a long 

 era of evolution,the simple primaryforms gave rise to a consider- 

 able variety of diverse animals, still comparatively minute in 

 size and simple in organization, swimming by means of cilia, 

 and typified to-day by the swimming embryos of invertebrate 



As yet — if our hypothesis is well founded — no life existed 

 upon the bottom of the seas, and the swimming forms were 

 destitute of any hard parts capable of fossilization. But why 

 did not some of these forms very early make their way to the 

 bottom and begin life under the new conditions of contact with 

 solid substance? And yet why should they have sought the 

 bottom ? Their food supply lay on or near the surface, the 

 bottom of the shallow waters may have been unsuitable 

 through the deposition of soft sediment, and the bottom of the 

 deeper waters very sparse in food. And, more important still, 

 they were quite unadapted to life on the bottom, and needed a 

 radical transformation before they could survive under such 

 conditions. If we look at the remarkable change which the 

 swimming embryo of a star-fish or sea-urchin, for example, 

 goes through before any resemblance to the mature form 

 appears, we may gain some idea of the long series of variations 

 which the primitive ciliated swimmers must have passed 

 through to convert them into crawling or stationary bottom- 

 dwelling forms. Great as was the period needed to produce 

 these type forms of life, another extended period must have 

 been necessary to convert them into well adapted habitants of 

 the solid floor of the seas. 



{To be Continued.) 



