502 HYATT'S REVISION OF THE 



increase toward the centre of development of the species, or toward the centre of devel- 

 opment of the group (genus, sub-family, or family, as the case may be) to which the spe- 

 cies belongs. This increase generally amounts to from five to twenty-five per cent, of the 

 average size of the species, but in very rare instances there is fovmd to be no appreciable 

 variation in size over^immense areas, or throughout the entire habitat of species having a 

 very wide geographical range. Since decrease in size so generally accompanies increase of 

 temperature, it may be considered as possibly due to the enervating, or other unfavorable 

 action of tropical influences upon the organism, a view strengthened by the fact that de- 

 crease in size is often accompanied by a loss of vivacity, and, in birds, a deterioration of 

 song. In the case of the exceptional increase in size southward in North American birds 

 and mammals, it is always found to occur in groups which attain their maximum develop- 

 ment, both as respects the number and the size of the species, in intertropical latitudes, 

 where of course such grou^is find their most fiivorable conditions of existence ; just as 

 increase of size northward occurs in sjiecies belonging to groups which have the greatest 

 number of species, and those of largest size, in the colder latitudes ; the increase in both 

 cases being in the direction of the region evidently most favorable for the existence of the 

 species. 



" That the above cited variations in size, in coloration, and in the size of particular 

 oi'gans, have direct relation to the conditions of environment, is conclusively shown by the 

 manner in which these variations occur. In many cases the extreme phases of differentiation 

 presented by a wide-ranging species are so diverse that they were often formerly regarded 

 as representing entirely distinct species, but subsequent material received from numerous 

 intermediate localities has shown a comj^lete and gradual intergradation between them ; so that 

 it has been found to be more in accordance with the facts to relegate many forms, formerly 

 and, as then known, properly regarded as distinct species, to the rank of geographical races 

 or sub-species. The passage from one extreme to the other, in any given case, is found to 

 be as gradual as that in the climatic conditions of the two climatically very diverse regions 

 under which these several phases reach their greatest specialization, and the majority 

 of the species occurring together over a wide area tend to run into similar local phases of 

 differentiation vmder similar conditions of environment." 



These results are fir too uniform and general in their effect upon widely distinct animals 

 to be due to Natural Selection, in fact, I entirelj' agree with Mr. Allen that such uniformity 

 of result here, and in the sponges, can only be explained by the action of physical sur- 

 roundings directly working upon the organization and producing by such direct action the 

 modifications or common variations above described. 



If a uniform result follows upon the exposure of a given animal, and its congeneric 

 forms, to the action of certain phj-sical conditions, the modifications which result must be 

 taken out of the category of those differences which may have been preserved by Natural 

 Selection, the proper field for which lies in the preservation of those differences which 

 arise through a tendency to variation, and are perpetuated and preserved simply because 

 they are of advantage to some particular animal or race. To assume that only those 

 species survive in these fields, the Mediterranean and others, whose organization fits them 

 to become modified in a particular way, is perhaps a fact which may be verified some day 

 by experiment, and in so far the law of the survival of the fittest probably obtains. 

 What I mean to point out is, that within this limit the animals are modified so directly. 



