108 



SCIENCE. 



[N. S. Vol. XXII. No. 552. 



time with their chief characteristics un- 

 changed, or they originated in some far 

 distant age of pre-Cambrian time and 

 reached their now known earliest stage of 

 evolution quite as slowly as they have 

 changed since they reached that stage. 

 Aside from the absence of evidence in favor 

 of the latter supposition, it is an improb- 

 able one in view of that extremely slow 

 increase of faunal rank in those subking- 

 doms which has just been mentioned. To 

 indicate that slow increase of faunal rank 

 we may -postulate a pair of chronological 

 lines extending from the Cambrian age to 

 the present time as the two sides of an 

 evolutional angle of parallax. The di- 

 vergence of such a pair of lines would be 

 so exceedingly small that the apex, which, 

 would represent the beginning of life, will 

 be carried back into the abyss of time to a 

 point inconceivably remote. It, therefore, 

 seems necessary to conclude that the earli- 

 est Cambrian faunas had a comparatively 

 sudden origin, notwithstanding the fact 

 that their development diverged so slowly 

 through subsequent time. 



A case partly similar to the one just 

 stated, but which is of narrower faunal 

 scope and shorter chronological range, is 

 presented by the Unionidse, the oldest cer- 

 tainly known fossil remains of which fam- - 

 ily are found in Triassic strata.^ Similar 

 remains are also found in successive for- 

 mations to the present time, living repre- 

 sentatives of the family being abundant in 

 the lakes and rivers of the world. The 

 genus JJnio, the typical member of that 

 generically small family, is thus known to 

 have existed continuously and unchanged 

 during all that stretch of time in which all 

 the mammals, all the birds, all the teleost 

 fishes, and all the exogenous plants of the 



^ Those Devonian shells which were referred by 

 Vahiixem to Anodonta, the several species of the 

 Naiadites of Dawson, and other paleozoic shells, 

 I do not now regard as belonging to the Unionidse. 



earth were introduced, and in which the 

 dinosaurs culminated and became extinct. 

 These earliest representatives of the Union- 

 ida3 are fully characteristic of the family, 

 but no certain trace of any previously ex- 

 isting member of it has been found in any 

 older strata. It seems necessary to as- 

 sume that this family, by its typical genus, 

 was suddenly introduced at that early 

 period, and that it has remained without 

 material change through all the subsequent 

 ages. ; 



One may suggest that the marine in- 

 vertebrates, which are represented by the 

 five lines under A on the diagram, have 

 preserved their faunal integrity through all 

 the geological ages because they had a con- 

 tinuous congenial marine habitat. But the 

 fresh-water invertebrates have survived 

 with quite as little change without such 

 obvious advantages. For example, the 

 Unionidge were from age to age subject to 

 shifting and adverse conditions resulting 

 from the changing relations of lands and 

 waters, and from the restriction and isola- 

 tion to M^hich all fresh waters are subjected 

 by intervening land. Their habitable 

 range was also restricted by the marine 

 waters into which their congenial fresh 

 waters flowed. If physical conditions of 

 environment were the dominant factors in 

 phylogenetic differentiation we should ex- 

 pect that these and other faunal groups of 

 fossil fresh-water mollusca would have be- 

 come widely and profusely differentiated. 

 As a matter of fact, however, those diverse 

 and frequently changing conditions did not 

 result in great differentiation of fresh 

 water faunas in general, a fact which is of 

 importance in this connection. On the 

 contrary, all fresh-water faunas are nota- 

 bly much less diversified than are marine 

 faunas; and fresh- water mollusca espe- 

 cially have retained their primitive types 

 through long geological ages under the 



