2IO THE HABITAT OF THE EURYPTERIDA 



lineal extent of rivers and the great struggle for existence, particularly 

 between crustaceous animals. For instance, Ortmann has pointed 

 out that freshwater crayfishes existed in Southeastern Asia, the Ma- 

 laysian Islands, India, and Madagascar in the Middle Cretacic. In 

 the Upper Cretacic the freshwater crabs (which are geologically 

 younger than the crayfishes) arrived (or originated) in Lemuria and 

 "extended into Southern Asia and the Malaysian Archipelago, 

 everywhere exterminating the crayfishes, namely, in India, South- 

 eastern Asia (Farther India and China) and on the islands. They 

 not only acted as a check to the distribution of the crayfishes, but 

 directly annihilated them" (Ortmann 201, 391). As a result, no 

 crayfishes are today found in the rivers of central and south Asia or 

 on the Malaysian Islands. 



We have previously seen that in river faunas the number of indi- 

 viduals is large but the number of genera and species is small, while 

 in marine faunas genera, species, and individuals are abundant. The 

 factor, then, of relative numbers of taxonomic groups would favor 

 marine organisms in widespread migrations. Pelagic and vagrant 

 benthonic organisms, living in the sea, have on the whole rather fa- 

 vorable conditions for migration. With river forms the factors of dis- 

 tribution are more accidental and much depends upon the individual. 

 In the region of interlacing headwaters, streams of different systems 

 are temporarily connected at times of flood and perhaps only two or 

 three individuals of a certain species will change from one system 

 to another, and then, when the connection is broken, the distribution 

 of that species depends entirely upon the ability of the individual to 

 contend with all of the new factors in the environment, and it is pure 

 survival of the fittest which brings about the distribution of that 

 species. In the sea, on the other hand, whole groups migrate or are 

 carried by currents, and the chances are good that a large number or 

 at least enough for populating a new region will survive, whatever 

 vicissitudes befall. Thus, to sum up, distribution of river forms over 

 broad areas is more precarious and fortuitious than is the case with 

 marine organisms. 



When we apply such considerations to fossil faunas, to a class of 

 organisms wholly extinct, where we have no facts of modern distri- 

 bution to help us, no facts of present habitat to point past modes of 

 life, we can see that the criteria which we apply to such fossil faunas 

 in the determination of relationships and migrations must be quite 

 different from the ones applied to marine fossil faunas. We can now 



