192 READINGS IN EVOLUTION, GENETICS, AND EUGENICS 



places and modified its flow It moulds its banks and bottom, 



forming here a bar, there an island, here a bay, there a point of land, 

 but still flowing on, though its course, its speed, its depth, the character 

 of the substances which its carries in suspension or in solution, all are 

 altered, built up by its own past activity." According to this view, 

 structure is simply the resultant of the interaction of function and en- 

 vironment or of functional activity. Though perhaps a little extreme 

 for most of us, this view is, we believe, essentially correct. We are 

 prone to overemphasize structure in our discussions of adaptation 

 and evolution and to lay too little stress upon the energy side of 

 development. Certainly no structure is ever formed without proto- 

 plasmic activity of a very definite sort, and in this sense adaptations 

 are to be thought of as the results of functioning. Why, then, do we 

 claim to be astonished at the effective way in which certain organs 

 accomplish their functions, when functioning has taught them their 

 task ? 



LAWS OF ADAPTATION 



Adaptations have been variously classified by different writers. 

 Perhaps the most significant classification is that of Osborn, which 

 is based on their supposed evolutionary origin. According to this 

 writer and others there are two categories of adaptations to environ- 

 mental conditions: the first has to do with the tendency of unrelated 

 species to assume similar structures under similar environmental 

 conditions; the second has to do with the tendency of related species 

 to assume different adaptive structures under different environmental 

 conditions. In both categories the environment appears to be the 

 determining factor. 



(i) A good example of the first category, which illustrates what 

 Osborn calls "the law of convergence or parallelism of form," is seen 

 in the tendency of many aquatic types of vertebrates to assume the 

 fishlike form. As is well shown in Fig. 40, the shark (a fish), the 

 ichthyosaur (an extinct aquatic reptile), and the porpoise (a marine 

 mammal), all possess the same fusiform body best adapted for speed 

 under water, the same types of locomotor structures, consisting of the 

 great propeller fin (caudal fin) and the steering and balancing fins, 

 the dorsal fins and paired fins. Apart from these superficial adapta- 

 tions for swift locomotion in the water, the three types are pro- 

 foundly different. The shark breathes with gills, the reptile and 

 mammal with lungs, the fish and reptile are cold-blooded, the 



