With one carte.
Professor Woltereck's name is associated, among other things, with the origin and differentiation of animal species in fresh-waters, and this account of some recent work by British investigators is contributed to his Festschrift to indicate how some of his ideas have found expression among workers in other parts of the world.
The metabolism of animals which have no mechanism for maintaining a constant body temperature, takes place more quickly in a warm environment than in a cold one. Accordingly we may look to the tropics to see life progressing most speedily. On the land, even in the tropics, there is often an arrest in metabolism every year or twice a year, as a consequence of dry seasons which are comparable to the winters of temperate latitudes. At these times growth is reduced, and reproductive activity is consequently restricted to certain times of the year. In the water environment, however, such a break in activity does not usually exist; growth is very rapid throughout the year, and there appears to be no definite breeding cycle depending on seasons. As a result, the generations pass more rapidly than in temperate latitudes, and therefore the change of bodily structure, which we call evolution, may also take place quicker. If we presume that species of fish for example, pass through a generation twice as quickly in the tropics as in a temperate zone, then a species which requires 5,000 generations to change into a new species, and passes through one generation in four years, would take 20,000 years l ) The object of this discussion is to explain a few of the problems and results in the study of Africa's freshwater fauna. The few facts summoned in illustration are not in themselves new, and they could be reinforced with many others by reference to the papers cited. I a m much indebted to Miss E. Trewavas of the British Museum (Nat. Hist.) for checking my remarks about the family Cichlidae and for certain unpublished data from her own work.
The periods involved in such reckoning 10,000, 20,000 or even 100,000 years, are of course very short on the geological time-scale, and the conclusion reached tentatively is that the differentiation of species and even of genera in tropical fresh-waters has taken place surprisingly fast.