190 The American Naturalist. [March, 



In April, 1885, 1 presented certain views on this subject 

 before the Academy of Natural Sciences of Philadelphia, and 

 reinforced my arguments by later communications in 1885 and 

 1886. In 1894 Professor W. K. Brooks, evidently unaware of 

 the existence of the papers mentioned, advanced a similar hy- 

 pothesis in the July-August number of the " Journal of 

 Geology," presenting a number of interesting facts, though 

 missing, as it seems to me, much the strongest argument in 

 defence of the hypothesis. 



I propose here to repeat my former hypothesis, with addi- 

 tional arguments and illustrations — for some of the latter of 

 which I acknowledge indebtedness to Professor Brooks's able 

 paper. 



To begin with, the facts of embryology may be said to point 

 directly to what was probably the primary condition of life. 

 The embryos of ocean animals, as a rule, begin life as swim- 

 ming forms. Even the oyster — a type of sluggishness in 

 animals — enjoys a brief existence as a swimmer before it ac- 

 quires a shell and becomes permanently fixed. The same is 

 the case with the sponge, the coral, and other stationary types, 

 and with the various creeping or slow moving forms, such as 

 the echinoderms. Since it has become a settled dogma of 

 science that each stage of development passed through by the 

 embryo represents some mature stage in the ancient ancestry 

 of the animal, the fact stated points almost irresistably to the 

 conclusion that the far off ancestors of the present stationary 

 or crawling animals were swimmers — and, for that matter, 

 naked swimmers', they being as yet destitute of hard skeletal 



Yet no swimming stage of existence is indicated by the old- 

 est known fossils, or at least only by the minute pteropods and 

 phyllopods, which were, perhaps, secondary derivatives from 

 crawling ancestors. The trilobite may have had some swim- 

 ming powers, yet probably made its way only by crawling, and 

 the other known forms were crawlers or burrowers, or were 

 immovably fixed. There are traces of jelly fish, it is true, but 

 these, as they now exist, we know to be derivatives from sta- 

 tionary forms, and the primeval swimmers indicated by em- 

 bryology have left no trace of their existence in the rocks. 



