Febkuary 16, 1906.] 



SCIENCE. 



245 



theory of natural selection, and more espe- 

 cially the idea that adaptations have arisen 

 because of their usefulness." 



Is not the fact that such diverse opinions 

 are held by thoughtful men of science in 

 itself an illustration of the pressure of en- 

 vironment? For is it not obvious— since 

 those who spend their lives studying species 

 in nature hold one view, while those who 

 spend their lives peering into microscopes 

 and inventing theories hold another view- 

 is it not obvious that such antagonistic 

 views are the outgrowth of different en- 

 vironments? 



To those who admit the existence of 

 dynamic influences it is conceivable that 

 the inception of the resulting variations, as 

 long ago pointed out by J. A. Allen, is not 

 confined to a single individual, to be lost or 

 saved by natural selection, but being the 

 response of organisms to forces applied 

 alike to all members of a species in a given 

 area, takes place simultaneously in a large 

 number of individuals and is thus estab- 

 lished more quickly and with much greater 

 certainty. 



VARIATIONS. 



It is co2ivenient for the systematist to 

 consider variations under four heads: (1) 

 Fortuitous variations; (2) dynamic varia- 

 tions; (3) sexual variations; and (4) sea- 

 sonal variations. 



1. Fortuitous variations are such as arise 

 without apparent cause — for it is in the 

 nature of organisms to vary. They origi- 

 nate in a single individual, and occur every- 

 where, in all species of animals and plants. 

 They may be beneficial, neutral or in- 

 jurious. The beneficial are likely to be 

 preserved by natural selection and in- 

 creased so long as the increase is beneficial 

 to the possessor; the neutral may either 

 disappear or persist ; the injurious are 

 promptly eliminated. According to degree 

 of development at the time of their first 

 appearance they may be designated ordi- 



nary individual variations, or sport varia- 

 tions. This is an old story. 



2. Dynamic variations are such as arise, 

 apparently, in response to pressure of the 

 environment. They are necessarily bene- 

 ficial, arise simultaneously in a number of 

 individuals, and are cumulative so long as 

 the increased development is helpful — that 

 is until a condition of equilibrium is estab- 

 lished between the organism and the en- 

 vironment. They may be functional or 

 geographic. Those that are functional 

 may be local or general; those that are 

 geographic may be local or progressive. 

 Variations resulting from change of food 

 habits, or from the necessity of coping with 

 a new enemy, or from the too rapid spread 

 of a species from one area to another, may 

 progress to full maturity in a given local- 

 ity; but in the case of ordinary geographic 

 variations the change progresses laterally 

 from one district to another. Thus in 

 species that undergo geographic color varia- 

 tions, the change as a rule takes place so 

 graduall}^ that specimens from adjacent 

 localities may be hard to distinguish, while 

 those from the two borders of the belt of 

 intergradation may be markedly unlike. 

 In this ease the intergrades, as well as the 

 extremes, are in complete accord with the 

 surroundings, the members of the species 

 at each step along the geographic line of 

 intergradation having attained a state of 

 equilibrium with reference to the environ- 

 ment at that point. 



Usually the movements of animals and 

 plants in acquiring new territory are slowy 

 allowing time for the necessary adjust- 

 ments to take place along the line of ad- 

 vance, so that no great change at any one 

 locality is required. But it is conceivable 

 that in certain cases the advance may be 

 too rapid for this, bringing a species into 

 an area to which it is not yet adapted, in 

 which case the struggle for existence would 

 be unusually severe and the development 



