NbVEMBEE 25, 1904.] 



SCIENCE. 



701 



with those of other, mammals, and not have 

 underrated the limitations of homology nor 

 the importance of the study of adaptations. 

 Only within the last few years have we 

 come to recognize that every orgai^ is more 

 than a homologue: it is also a successful 

 experiment with the environment. 



The existence of that relation between 

 form and environment which we call adap- 

 tation has been recognized for centuries, 

 yet its full significance is still obscure. 

 The prevailing theory (of Darwin) as- 

 sumes that a change in environment pre- 

 cedes any change in form and that adapta- 

 tion is, therefore, necessarily achieved by 

 a change -in the mean of the form to meet 

 the changed demands of the new environ- 

 ment. This theory may, indeed, be said 

 to be the natural outcome of the morpho- 

 logical doctrine of fundamental fixity of 

 type. The type could be bent to meet new 

 conditions, but could receive nothing new 

 nor suffer loss of parts. I find that in the 

 pre-Darwinian epoch Prichard suggested 

 that the creator made the various species 

 and placed them in habitats for which their 

 structure fitted them. We see in this sug- 

 gestion translated into modern terms the 

 germ of a very different theory of adapta- 

 tion from the prevailing one. I expressed 

 this nearly two years ago about as follows : 

 "The world contains numberless kinds of 

 habitats or environmental complexes ca- 

 pable of supporting organisms. The num- 

 ber of kinds of organisms is very great; 

 each lives in a habitat consonant with its 

 structure." Each species is being widely 

 dispersed while, at the same time, it is 

 varying or mutating. By chance, some 

 variants of the species get into an environ- 

 ment worse fitted for them; others into 

 one better fitted. "Those that get into 

 the worse environment can not compete 

 with the species already present ; those that 

 get into a habitat that completely accords 

 with their organization will probably thrive 



and may make room for themselves, even 

 as the English sparrow has made room for 

 itself in this country. This process may 

 go on until the species is found only in 

 the environment or environments suited to 

 its organization. As Darwinism is called 

 the survival of the fittest organisms, so 

 this may be called the theory of segrega- 

 tion in the fittest environment. ' ' 



The principle that animals are found in 

 habitats for which their structure fits them, 

 and not elsewhere, points to the close re- 

 lation existing between morphology and 

 geography. We find the animals of the 

 seashore, such as sponges, Hydro- and An- 

 thozoa, and tunieates, to be largely sessile, 

 and in consequence, of the radiate type of 

 structure. This sessile habit makes it pos- 

 sible for them to maintain their hold on 

 the rocks from which the beating waves 

 tend to tear them. Those which are not 

 actually permanently attached have means 

 enabling them to cling closely to the rocks ; 

 such are the echinoderms, the molluscs, 

 many annelids and crustaceans. The ani- 

 mals of the surface of the sea, such as 

 siphonophores, ctenophores, jelly fishes and 

 larvte, are without such clinging organs; 

 they include species whose organization 

 permits them only to float or swim at or 

 near the surface. The deep sea could, have 

 been populated more readily, so far as 

 proximity goes, from the surface organ- 

 isms than from those of the shore line ; but 

 only the latter ofi'ered the structural fea- 

 tures consonant with life at the sea bottom 

 and so the deep sea became populated 

 thence. In the swift-flowing rivers we 

 find powerful swimmers or animals that 

 can hold fast to the bed of the stream and 

 in ponds we find those species which have 

 some means of preserving their continuity 

 in time of drought. In caves we get not 

 any forms which happen to be washed into 

 them, but only darlmess, moisture- and con- 

 tact-lovers. In deserts whole groups of 



