PROBLEM II 395 



species in the Wenlock have a range in the Silurian rocks of England as 

 great at least as the whole series of beds represented in the Silurian of 

 the Eastport region, and some of them appear both in Ordovician and 

 Devonian strata. 



Supposing we accept the proposition that the order of succession of 

 the fauna points correctly to a gradual lifting of the sea bottom to the 

 surface in the two areas, is not the testimony as strong evidence that the 

 several faunas indicate only like conditions of environment as that they 

 indicate contemporaneous periods of time? 



Which of these two conclusions we adopt will depend in great measure 

 on which of two diverse hypotheses we accept regarding the causes of 

 change in form of organisms. For convenience, call these two the hy- 

 pothesis of migration and the hypothesis of evolution. 



Hypothesis of migration. — By the hypothesis of migration I mean 

 that in seeking an explanation for the change in composition of the fossil 

 faunas we adopt the hypothesis that the changes in the environmental 

 conditions under which the fauna of the lower formation lived resulted 

 in the departure of that fauna from the area under examination, and a 

 new set of species adjusted to the new conditions migrated into the area. 



In this hypothesis several propositions regarding the relations of the 

 morphology of species to conditions of environment are involved. 



1. It is assumed that a species maintains its morphologic characters 

 and adjustment to conditions of life, and when in. one locality the habitat 

 becomes unfavorable the species migrates or moves out to find more con- 

 genial conditions or dies. 



2. There is also involved the assumption that the overlying fauna was 

 already nourishing elsewhere during the time of occupation of the area 

 by the lower fauna, which means that although in the geological section 

 the one fauna makes its record later than the other the two are actually 

 contemporaneous faunas adjusted and living under diverse physical con- 

 ditions at the same time. 



And these two propositions appear to be well founded on observed 

 facts in present-time conditions — that is, we know that a( the present 

 time there are living in the oceans quite diverse fauna! combinations of 

 species — more or less closely adjusted to differences of depth in the 

 water, different degrees of purity and density of the salt water, differences 

 of temperature, and also limited to particular geographical provinces of 

 distribution. 



We seem, therefore, to be fully justified in assuming that a similar 

 set of conditions prevailed in Paleozoic time. 



