10 JOEL ASAPH ALLEN— CHAPMAN IMemoiM [vouxxi: 



The subject of geographic variation continued actively to hold Doctor Allen's attention 

 and, five years after the appearance of the Museum of Comparative Zoology essay, he published 

 a paper on " Geographical variation among North American mammals, especially in respect to 

 size," in which the following laws in regard to fissiped carnivora were enunciated: 



(1) The maximum physical development of the individual is attained where the conditions of environment are 

 most favorable to the life of the species. Species being primarily limited in their distribution by climatic condi- 

 tions, their representatives living at or near either of their respective latitudinal boundaries are more or less 

 unfavorably affected by the influences that finally limit the range of the species. . . . 



(2) The largest species of a group (genus, subfamily, or family, as the case may be) are found where the group 

 in which they severally belong reaches its highest development, or where it has what may be termed its center of dis- 

 tribution. In other words, species of a given group attain their maximum size where the conditions of existence 

 for the group in question are the most favorable, just as the largest representatives of a species are found where 

 the conditions are most favorable for the existence of the species. 



(3) The most 'typical' or most generalized representatives of a group are found also near Us center of distribu- 

 tion, outlying forms being generally more or less 'aberrant' or specialized. Thus the Cervida?, though nearly 

 cosmopolitan in their distribution, attain their greatest development, both as respects the size and the number 

 of the species, in the temperate portions of the northern hemisphere. The tropical species of this group are 

 the smallest of its representatives. Those of the temperate and cold temperate regions are the largest, where, 

 too, the species are the most numerous. . . . The possession of large, branching, deciduous antlers forms 

 one of the marked features of the family. These appendages attain their greatest development in the northern 

 species, the tropical forms having them reduced almost to mere spikes, which in some species never pass beyond 

 a rudimentary state. 



A year later he contributed to the Radical Review (May, 1877, pp. 108-140) an article on 

 "The influence of physical conditions in the genesis of species," which is fundamentally so 

 sound and logical that 29 years later the Smithsonian Institution requested permission to 

 republish it (Ann. Rep. for 1905, pp. 375-402). 



Doctor Allen here contended that the direct modifying influences of environment are more 

 potent factors in evolution than natural selection, taken in the narrow sense of the "survival 

 of the fittest." Climate is shown to be the most active agent in promoting variations in size 

 and in color, but habits and food and the geological character of the country are considered 

 to play their part. Of the action of climatic influences he wrote: 



That varieties may and do arise by the action of climatic influences, and pass on to become species, and that 

 species become, in like manner, differentiated into genera, is abundantly indicated by the facts of geographical 

 distribution and the obvious relation of local forms to the conditions of environment. The present more or 

 less unstable condition of the circumstances surrounding organic beings, together with the known mutations of 

 climate our planet has undergone in past geological ages, points clearly to the agency of physical conditions 

 as one of the chief factors in the evolution of new forms of life. So long as the environing conditions remain 

 stable, just so long will permanency of character be maintained; but let changes occur, however gradual or 

 minute, and differentiation begins. If too sudden or too great, extinction of many forms may result, giving 

 rise to breaks in the chain of genetically connected organisms. 



Due allowance, however, he states must be made for relative plasticity or susceptibility 

 to the influences of environment shown by closely allied species. 



He also considers the possibility of species arising through what has since become known 

 as mutation ; -writing : 



But it is supposed, again, that new forms are not always thus gradually evolved from minute beginnings, but 

 sometimes — perhaps not infrequently — arise by a saltus; that individuals may be born widely different from 

 their parents, differing so widely and persistently as not to be so readily absorbed by the parental stock. In 

 proof of this, instances are cited of new species apparently appearing suddenly, and of varieties thus originating 

 under artificial conditions resulting from domestication. Granting that new forms may thus arise, although 

 as yet few facts have been adduced in its support, they are necessarily at first local, and in no way accord with 

 the observed geographical differences that characterize particular regions, and which affect similarly many 

 species belonging to widely different groups. 



Meanwhile, Doctor Allen was pursuing on a larger scale the studies in distribution, of which 

 the earlier results were announced in the fifth part of the Mammals and Winter Birds of East 

 Florida, and in 1878 there appeared his paper on " The geographical distribution of mammals, 

 considered in relation to the principal ontological regions of the earth and the laws that govern 

 the distribution of animal life." (Bull. U. S. Geol. Surv. IV, No. 2, pp. 313-377.) 



