250 ANIMAL MORPHOLOGY 



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 

 organ 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 adaptation has been recognized for centuries, yet its full sig- 

 nificance is still obscure. The prevailing theory (of Darwin) assumes 

 that a change in environment precedes any change in form and that 

 adaptation 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 morphological 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 Prich- 

 ard * suggested that the Creator made the various species and placed 

 them in habitats for which their structure fitted them. We see in this 

 suggestion translated into modern terms the germ of a very different 

 theory of adaptation from the prevailing one. I expressed this nearly 

 two years ago about as follows: 2 "The world contains numberless 

 kinds of habitats or environmental complexes capable of supporting 

 organisms. The number 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 cannot 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 segregation in the fittest environment." 



The principle that animals are found in habitats for which their 



1 J. C. Prichard, Physical History of Mankind, 3d ed., 1836-37, vol. i, p. 96: 

 " The various tribes of organized beings were originally placed by the Creator in 

 certain regions, for which they are by their nature peculiarly adapted." I owe 

 this reference to F. Darwin's More Letters 0} Charles Darwin, vol. i, p. 45, New 

 York, Appleton. 



2 The A nimal Ecology of the Cold Spring Sand Spit, with remarks on the Theory 

 of Adaptation, The Decennial Publications of the University of Chicago. Preprint 

 dated Jan. 1, 1903. This was written in the autumn of 1902. The sentences 

 quoted are on page 21 . The same idea is worked out in my paper, The Collembola of 

 Cold Spring Beach, with special reference to the movements of the Poduridie, Cold 

 Spring Harbor Monographs, n, pp. 24, 25, July, 1903. In T. H. Morgan's book, 

 Evolution and Adaptation, this view is adopted. This book was published in 

 November, 1903. 



