PHYSICAL CONDITIONS IN GENESIS OF SPECIES. 399 



under the stinmlating climate of the region drained by the upper 

 Mississippi and npper Missouri rivers, being, as a rule, hirger here 

 than in corresponding latitudes more to the eastward. The decrease 

 .in size toward both the northorji and southern borders of the habitat 

 of a giA'en species or genus, of Avhich there are many marked in- 

 stances, further shows that size varies with the var^dng conditions 

 of habitat and reaches the maximum only where the conditions are 

 most favorable to the life of the species. 



Much has been written respecting the influence of climate on man, 

 and many speculations have been indulged in in relation to the part 

 the conditions of life have taken in bringing about the diversity at 

 present existing among the different races. A striking parallelism 

 is often observable between the leading features of geographical 

 variation among animals and the physical differences that obtain 

 among nations or races of men inhabiting the same areas and sub- 

 jected to the same influences. While civilized man is, in a measure, 

 less the subject of such influences than the lower animals, he is not 

 wholly above them. Certain regions more favor both physical and 

 intellectual development than others, and these prove to be, as would 

 be expected, the milder temperate portions of the globe, where the 

 struggle for a mere vegetative existence is reduced to a minimum. 



The influence of different climatic conditions upon members of the 

 same nationality find exemplification in different parts of our own 

 coimtrv, and are so obvious as to be the subject of frequent obser- 

 vation and comment. The same original stock is found to gradually 

 develop certain peculiar physical and mental characteristics when 

 placed imder diverse conditions of climate, certain localities more fa- 

 voring intellectual growth and activity than others, just as certain 

 regions are characterized by the frequent occurrence of particular dis- 

 eases which in other regions are exceptional. While humiditj'^ and a 

 high temperature, Avhen combined, are found to be enervating and de- 

 teriorating, a clear, dry atmosphere favors vigor of both mind and 

 body. But the subject of the influence of climatic conditions upon 

 man is too vast to be entered upon in detail in the present connection. 

 The study of man from a geographical standpoint, or with special ref- 

 erence to conditions of environment, offers a most important and fruit- 

 ful field of research, which, it is to be hoped, will soon receive a more 

 careful attention than has as 5'et been given it. 



In conclusion, a few Avords seem called for concerning the ques- 

 tion. What is a species? as well as in respect to the bearing of the 

 general facts above detailed upon the evolution of specific forms. 



As is well known, the belief that siDccies were distinct and immut- 

 able creations was long the prevailing one among naturalists. Yet 

 the question of what constitutes a species is one about which endless 



