88 The American Naturalist [January, 



which occur in mammals and birds, as those which occur in the lower 

 forms — reptiles and fishes — are less well known. He said that a rather 

 wide range of individual variation is recognized as inherent in all ani- 

 mals. These variations, however, are usually confined within rather 

 narrow constant limits, any considerable excess beyond the normal 

 range coming into the category of sports and are popularly termed for- 

 tuitous. A step further gives malformations and monsti osities. Such 

 extreme departures from the normal, while more rare, are probably no 

 more fortuitous than those less marked. The only difference was that 

 the immediate cause was hard to discover. 



He illustrated this point by a number of examples, in which changes 

 were plainly due to geographical and climatic forces. As a result 

 biologists had accepted certain generalizations which might be stated 

 as follows : 



First. Baird's law of geographical variation in size, which, 

 announced in 1865, still held its own. It asserted that there was a 

 constant increase in the size of individuals of the same 'species from 

 the south northward, and from the lowlands toward the higher eleva- 



Second. The frequent increase in size of peripheral parts, as the 

 tail, beak and claws of birds, took place from the north southward or 

 inversely to the increase and general size. 



Third. A general deepening of the coloration took place from the 

 north southward in North America, east of the plains, together with a 

 reduction of white markings and white areas, and a corresponding 

 increase of dark markings and dark areas and a gradual increase in 

 the intensity of iridescent tints in species thus marked. 



Fourth. The loss of color over the interior in both mammals and 

 birds having a continental distribution is marked. 



Fifth. There is an extreme intensification of color over the heavy 

 rainfall district of the Northwest coast. 



The failure of verification of the first law led to its modification to 

 the following formulas, first published in 1876, which has stood the 

 test of subsequent investigation : 



First. The maximum physical development of the individual is 

 attained where the conditions of environment are most favorable to 

 the life of the species. 



Second. The largest species of a group (genus, sub-family or family, 

 as the case might be) are found where the group to which they sev- 

 erally belong reaches its highest development, or where it has what 

 may be termed its centre of distribution. 



