lineal extent of rivers and the great straggle for existence, particularly between crastaceous animals. For instance, Ortmann has pointed out that freshwater crayfishes existed in Southeastern Asia, the Malaysian 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 individuals 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 favorable conditions for migration. With river forms the factors of distribution 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 distribution 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
