not constant for any one body of water for any length of time, owing to seasonal and other changes. An organism adapted equally to any one of the 500, if that were physically possible, could not be closely adapted to any, and would be perhaps more or less inefficient under all. An organism adapted exclusively to one or two of the 500 could not in practise confine itself to them, and would be in danger of extermination. There would accordingly arise an optimum condition of adaptability, according to which any given organism would exist under at least 300 of the 500 postulated conditions, would do well under 100, and would flourish exceeedingly well under perhaps 10. Hence the species would be very widespread, would often be common, and would occasionally occur in excessive numbers; which is approximately what we find.
All of this would require in the animals much stability of type. If they varied freely and indiscriminately, the variations being inherited, they would not only tend to lose their standards of efficiency, but the selective processes might make playthings of them, changing them temporarily to meet this or that condition, but rarely able to reverse quickly enough as conditions altered.
The rhizopodous genus Difflugia contains a great number of species, differing in the size and shape of the little shells they make. It is not necessary to suppose that each species is specially adapted to some particular set of conditions, though some of them may be more or less so. It is only necessary to suppose that the difflugian type reached in these animals so many "positions of organic stability," which persisted and survived simply for this reason. There is a "survival of the fittest" in inorganic chemical compounds, following analogous lines.
There is the greatest contrast between these fresh-water protozoans and some of the marine groups, particularly the Radiolaria. Haeckel's great Challenger report on the radiolarians only partially indicates the enormous diversity of skeletal structure in these animals. They remind us more of snow crystals than anything else and it is useful to remember here that snow crystals, with all their wonderful diversity, are simply . It is impossible to believe that all this radiolarian diversity can be adaptive in more than the most general way; we would rather believe that it is possible because of the relative simplicity and uniformity of the conditions of life, which permit infinite diversity in the details of skeletal structure without injury. There is perhaps a high degree of stability in the protoplasmic structure of the radiolaria, and the modifications in the skeletons or shells may not indicate much fundamental diversity. To what extent the described species are permanent and constant is not known.
In multicellular animals, conditions are in many ways different; yet even here we notice a remarkable limitation in the types of cellular
