244 



SCIENCE. 



[N. S. Vol. XXIII. No. 581. 



acters by which one species is actually 

 known from the next are rarely traits of 

 utility.'^ 



This was Darwin's view also, for he says: 

 "It is a strange result which we thus ar- 

 rive at, namely, that characters of slight 

 vital importance to the species are the most 

 important to the systematist. ' ' T. H. Mor- 

 gan goes even further, .expressing his en- 

 tire disbelief in the production of species 

 through natural selection. Had he been 

 familiar with the characters that distin- 

 guish many of our species of mammals, 

 birds and reptiles he could hardly have 

 held this view, for while in some gi'oups 

 the characters naturalists make use of in 

 defining species are mainly of the indiffer- 

 ent kind, in others they are largely of the 

 adaptive and useful kind. Thus among 

 mammals, birds and reptiles dozens of 

 species and si^bspecies are distinguished 

 from one another by adaptive characters 

 alone. At the same time it should be 

 understood that, as a rule, species are 

 based not on one but several characters, 

 some of which may be adaptive, others 

 non-adaptive. 



PRESSURE OP THE ENVIRONMENT: 



Dar^vin's definition of natural selection 

 or the survival of the fittest is: "The pres- 

 ervation of favorable individual differences 

 and variations and the destruction of 

 those that are injurious." 



Whether a particular variation is bene- 

 ficial or injurious depends of course on 

 the environment. It follows that varia- 

 tions in order to be beneficial must be in 

 harmony with, or meet demands of, the 

 environment. Whether such adaptations 

 should be attributed wholly to the slow 

 action of natural selection, or whether one 

 may be allowed to cherish the belief that 

 the environment not only invites but com- 



"SCTENCE, N. S., Vol. XXII., p. 558, Nov. 3, 

 1905. 



pels variation, involves a distinction as im- 

 portant as it is difficult of demonstration. 

 Is there not a hidden force which independ- 

 ent of, or superadded to, the general tend- 

 ency of organisms to vary acts as an in- 

 centive in initiating useful variations? 

 Have we not evidence of this in the in- 

 creased length of toes and feet in species 

 inhabiting soft or marshy ground ; in- 

 creased thickness of bill in birds that 

 habitually crack seeds or nuts ; increase 

 in the length of the canine teeth in mam- 

 mals that catch and hold struggling prey j 

 increase in the size of the carnassial teeth 

 in mammals that crunch bones of other 

 animals ; special modifiications of the feet 

 to fit them for special purposes, as for 

 wading, swimming or grasping; Eind also 

 such variations as appear to result from 

 prolonged and oft-repeated effort, as in- 

 creased length of leg in species whose wel- 

 fare depends on fleetness of foot, and 

 greater development of the external ear, or 

 greater complexity of the internal auditory 

 apparatus which seem to be a consequence 

 of long continued strain in listening for 

 the approach of enemies or of prey? This 

 siibtle influence I like to call the pressure 

 of the environment. 



Dall was once bold enough to assert: 

 "The environment stands in relation to 

 the individual as the hammer and anvil to 

 the blacksmith's hot iron. The organism 

 suffers during its entire existence a con- 

 tinuous series of mechanical impacts, none 

 the less real because invisible. ' ' * These are 

 weighty words, well worth remembering. 



AVhile a number of our ablest natural- 

 ists hold this view, the current feeling 

 among morphologists and physiologists is 

 undoubtedly against it. Thus Thomas 

 Hunt Morgan in his book on 'Evolution 

 and Adaptation' (1903), says: "We can 

 profitably reject, as I believe, much of the 



* ' Dynamic Influences in Evolution,' W. H. Dall, 

 Proc. Biol. Soc. Wash., VI., 2, 1890. 



