THE BACKGROUND OF DARWINISM: ADAPTATIONS 211 



have the same period of activity and the same general habits. The 

 model must be much more numerous than the mimic. Unfortunately 

 for the proponents of mimicry, it has sometimes been found that 

 several of these requirements are lacking. Attempts to explain away 

 the discrepancies have been far from satisfactory. 



All these considerations should make us cautious about reading 

 into the colors and patterns of animals too many adaptive details. It 

 is more than likely that the majority, if not all, of these apparently 

 marvelously exact instances of imitative resemblance would turn out, 

 when critically examined, to be no more nor less adaptive in special 

 ways than is Kallima and the mimics. 



One lesson that the naturalist may well learn from the present 

 discussion is this: There is enough in the way of adaptations for the 

 evolutionist to explain without burdening him with hypothetical or 

 interpretative adaptations. First find and prove your adaptation; 

 then try to explain it. Don't explain it first and then find out later 

 that it was not so much of an adaptation after all. 



osborn's 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. 42, 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 steermg 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 



