August 26, 1898.] 



SCIENCE. 



249 



at the same time the most interesting, prob- 

 lem in the natural history of the earth." * 



Still more recently f he remarks : 



" Whenever the physical or organic con- 

 ditions change to however small an extent, 

 some corresponding change will be produced 

 in the flora and fauna, since, considering 

 the severe struggle for existence and the 

 complex relations of the various organisms, 

 it is hardly possible that the change should 

 not be beneficial to some species and hurt- 

 ful to others." 



Two conclusions are now generally ac- 

 cepted. The first is, that the most complete 

 evidence of evolution is aflForded by paleon- 

 tology. Huxley's vigorous affirmation, that 

 the primary and direct evidence in favor of 

 evolution can be furnished only by paleon- 

 tology, has been greatly strengthened by 

 recent discoveries. The second is that 

 biological evolution has been primarily de- 

 pendent on physical and geological changes. 



It may not be unprofitable for us as zo- 

 ologists to pass in review some of the revo- 

 lutions in geological history, particularly as 

 regards our own continent, some important 

 details of which have recently been worked 

 out by our geologists, and to note the inti- 

 mate relation between these revolutions and 

 the origination not only of new species, but 

 of new faunse, and, indeed, at certain epochs, 

 of new types of organic life. 



1. Precamhrian revolutions. That im- 

 mensely long period which intervened be- 

 tween the time when our planet had cooled 

 down and become fitted for the existence of 

 animal life, and the opening of the Cam- 

 brian period, was evidently a time of the 

 geologically rapid production of ordinal and ' 

 class types of invertebrate life. This is 

 strongly suggested by the fact that a large 

 proportion of the Cambrian classes embrace 

 forms as highly specialized as their succes- 

 sors of the present day, so that we are com- 



* Natural Selection, p. 14. 

 t Darwinism, 1889, p. 115. 



pelled to look many ages back of the Cam- 

 brian for the appearance of their generalized 

 ancestral forms. 



Of the eight branches, of phyla, of the 

 animal kingdom, the remains of seven, or 

 all except the vertebrates, have been found 

 in Cambrian strata. Adopting the kind of 

 statistics employed by Professor H. S. "Wil- 

 liams in his admirable Geological Biology, 

 but with some changes necessitated by a 

 little diiferent view as to the number of 

 classes living at the beginning of the Cam- 

 brian period, it appears that 13 out of 26 

 classes of the animal kingdom, occurring in 

 a fossil condition, already existed in the 

 Cambrian and, if we throw out from the 

 vertebrate classes those without a solid 

 skeleton (the Enteropneusta or Balanoglos- 

 sus, Tunicates, Amphioxus and the lam- 

 preys) 13 out of 22. Also, if we exclude 

 the land forms (Arachnida, Myriopoda and 

 insects), 13 out of 19, and then throwing 

 out the five vertebrate classes found in a 

 fossil state, of 14 invertebrate marine 

 classes 13 occur in the Cambrian.* With 

 little doubt flat-worms, nemerteans, Nema- 

 telminthes and Gephyrea existed then, and 

 probably the representatives of other 

 classes, of which no traces will ever occur. 



We shall for our present purpose follow 

 the classification of the U. S. Geological 

 Survey and restrict what was formerly 

 called the Archean to the fundamental 

 gneiss and crystalline schists of an unknown 

 thickness, and accept the Algonkian, as 

 comprising the Huronian and Keeweena- 

 wan formations. We may assume that the 

 first beginnings of life took place toward 

 the end of the Archean and that the more 

 or less rapid differentiation of class types 

 went on during Algonkian time. This 

 view is fortified by the statement of Wal- 



* Should the Polyzoa be traced to the Cambrian, as 

 is not at all impossible, the fact would remain that 

 every class of marine invertebrates with solid parts is 

 represented in the Cambrian. 



