FAUNAL RADIATION 341 



lias been largel}'^ determined by the climatic variations and the geologic 

 changes of the past. 



A¥e all now know that geologic time is very, very long; and yet if the 

 present geographic relations and topographic form, combined with the 

 ])resent climates, were to remain michanged, it is thought that there 

 \vould result almost no organic evolution. In other words, with the pres- 

 ent environment retained and in the absence of the destructive tendencies 

 of man, the organic world of today Avould continue almost unchanged. 

 However, we geologists know that the earth's land surface will not long- 

 remain as it is today. The beautiful mountains flow piecemeal down the 

 rivers into the seas and oceans, and thus extend the land areas, but peri- 

 odically the land surfaces are raised high into the atmosphere, through 

 whose agencies they were again and again destroyed. In consequence of 

 these geologic changes, the climates and environments are repeatedly 

 changed, and new routes of organic migration are established through the 

 making of land bridges which unite the continents, or the latter become 

 dismembered through doAvn-sinkings of the land areas and thus open up 

 new marine portals. In the one case, the life of the land spreads, while 

 that of the oceans is localized; and in the other it is just the reverse 

 condition which obtains. 



Faunal Eadiation 



If all the species of a fauna were restricted to a radiation of at most 

 one or two hundred miles, it would be exceedingly difficult to work out 

 zoogeographic provinces and realms, and in stratigraphy the faunal 

 realms probably could not be ascertained at all because of our present 

 scattering paleontologic knoAvledge. Since, however, at least 5 per cent 

 of living marine invertebrates have coastline ranges of from 5,000 to 

 upAvard of 10,000 and even 15,000 miles, and since about 60 per cent of 

 bottom-dAvelling forms have ranges of betAveen 2,000 and 3,000 miles, 

 this variably Avide distribution shows with certainty that the provinces 

 and realms of the geologic past can also be ascertained. 



It is this variable range of most of the marine invertebrates, and the 

 additional fact that nearly all organisms are changing, some very sloAvly 

 or seemingly not at all and others faster, that enable paleontologists to 

 work out the times of their geologic origin, duration, and vanishing. 

 Organic eA'olution is in the main progressive and in the direction of 

 greater complexity, though a small per cent of the species are stationary 

 or even regressive; and yet, in all this change, no species or genus is re- 

 peated in time. These facts indicate that fossils can be depended on in 



