206 EVOLUTION AND ANIMAL LIFE 



Others, not questioning the reality of the direct effects of 

 environmental forces on the individual, find no evidence that 

 these impacts are perpetuated in heredity. 



Besides direct effects of these outside influences, we have to 

 consider an infinite variety of reactions, which these forces or 

 impacts may set up in the organism. These again have been 

 supposed to be hereditary, for the species changes under them 

 in what seems to be very much the same fashion that the indi- 

 vidual does. Use and disuse in the species bring about parallel 

 results to those shown by use and disuse in the individual, and 

 are by some therefore referred to the same cause. 



But again there is grave reason to question the fact of the 

 inheritance of such reactions, and to doubt whether the effects 

 of use and disuse in the species rest on the same set of causes as 

 the results of use and disuse in the individual. 



There remains the supposition, adopted at least tentatively 

 by a large proportion, probably the majority, of the naturalists 

 of to-day, that the direct effects of environment, as well as the 

 reactions or indirect effects on the individual, are not repeated 

 in heredity, and that the selective influence of environmental 

 causes is the measure of their influence in the transformation of 

 species. 



The question of the nature of dynamic forces in evolution is 

 one of the most recent and most interesting phases of the long- 

 continued discussion of the inheritance of acquired characters. 



A vast range of variations are ontogenetic, or dependent on 

 influences affecting directly the life of the individual. These 

 ontogenetic variations are, strictly speaking, individual, ap- 

 pearing as collective only when many individuals have been 

 subjected to the same conditions. They may be divided into 

 environmental variations and functional variations, two cate- 

 gories which cannot always be clearly separated, as variations 

 due to food conditions partake of the nature of both. 



More than thirty years ago, Dr. J. A. Allen demonstrated 

 that climatic influences affect the averages in measurements and 

 in color among birds. For example, in several species of birds, 

 the total length is greater in specimens from the north, while 

 the bills and toes are actually longer in southern specimens. 

 That this condition is due to the influence of climate on develop- 

 ment is apparently shown by the fact that numerous species are 

 affected in the same way. It is noticed also that specimens 



