252 



SCIENCE 



[N. S. Vol. XXXI. No. 790 



development, such as infancy, youth and 

 old age, wlien these limits are narrower 

 than they are for the epoch of full vigor- 

 ous development of the adult; and these 

 sensitive periods are those which are 

 critical for the history of the species. 



The influences which govern evolution 

 have recently been stated by two of our 

 great biologists, Jordan and Osborn. They 

 both recognize that in the development of 

 organic life the grip of environment holds. 



Jordan," dwelling on the importance of 

 isolation as a factor in evolution, recently 

 wrote : 



Among tlie factors everyAvliere and inevitably 

 connected with the course of descent of any spe- 

 cies, variation, heredity, selection and isolation 

 must appear; the first two innate, part of the 

 definition of organic life, the last two extrinsic, 

 arising from the necessities of environment, and 

 not one of these can find leverage without the 

 presence of each of the others. 



Osborn has put the same principle as fol- 

 lows :^° 



The life and evolution of organisms continu- 

 ously center around the processes which we term 

 heredity, ontogeny, environment and selection; 

 these have been inseparable and interacting from 

 the beginning; a change introduced or initiated 

 through any one of these factors causes a change 

 in all. First, that while inseparable from all the 

 others each process may in certain conditions be- 

 come an initiative or leading factor; second, that 

 in complex organisms, one factor may at the same 

 time be initiative to another group of characters, 

 the inseparable action bringing about a continu- 

 ously harmonious result. 



These modern statements of the law of 

 natural selection find application immedi- 

 ately as we contemplate the procession of 

 geographies. Change of environment is 

 inherent in the movement of the procession 

 down the ages and, cooperating with in- 



^ Jordan, David Starr, Isolation as a Factor in 

 Organic Evolution, in " Fifty Years of Darwin- 

 ism," 1909, pp. 90-91. 



'° Osborn, Henry Fairfield, Darwin and Paleon- 

 tology, in " Fifty Years of Darwinism," 1909, pp. 

 238-239. 



trinsic biotic forces, has caused modifica- 

 tion of organisms as a necessary conse- 

 quence. 



Environment as related to any species or 

 to any flora or fauna may be said to be that 

 combination of conditions to which the 

 fauna is adapted and beyond which it can 

 not range into other environments. From 

 this follows the principle: Except through 

 reneived adaptation, an adapted fauna can 

 migrate only as the limits of its haiitat re- 

 cede, as the area of its environment broad- 

 ens. 



To apply this principle to the distribu- 

 tion and migration of species or groups of 

 species under the general law of periodic- 

 ity, we may follow the course of a cycle of 

 changes, from an epoch of diverse condi- 

 tions through a cosmopolitan state to di- 

 versity again. 



Diverse conditions of any one geographic 

 state may have been grouped simultane- 

 ously to form many environments or 

 f aunal provinces, and each of these has then 

 been occupied by its peculiar fauna con- 

 temporaneously with more or less unlike 

 faunas in other provinces. Each of these 

 faunas represented an adaptation to the 

 conditions of its peculiar environment. 

 The peculiarities of other faunal provinces 

 surrounding it constituted barriei-s beyond 

 which the species could not live, or could 

 not rear their young, even if the adults 

 could exist under the adverse conditions. 

 Only mthin those barriers could those 

 specially adapted species long continue to 

 exist. If their habitat became contracted 

 they also must contract their range; if it 

 shifted or expanded, they might migrate 

 accordingly. And there would be corre- 

 sponding migration or restriction of faunas 

 which were diversely adapted. Any cause 

 which shifted the conditions of light, heat 

 or food, brought opportunity to some, 

 death to others. 



