NATURAL SELECTION 391 



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 exter- 

 mination. 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 tem- 

 porarily 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 par- 

 ticular 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 Eadiolaria. Haeckel's 

 great Challenger report on the radiolarians only partially indicates the 

 enormous diversity of skeletal structure in these animals. They re- 

 mind 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 H 2 0. 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 xadiolaria, 

 and the modifications in the skeletons or shells may not indicate much 

 fundamental diversity. To what extent the described species are per- 

 manent 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 



