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 
environment, 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 P 
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. 
(rz) 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 
