INTERNAL CAUSES OF VARIATION 217 



Reversion and atavism are but special instances under the 

 law of ancestral heredity to be discussed later, serving to show 

 that inheritance is partly from ancestors back of the parent. 



Certain internal factors are of such nature as to affect the 

 race as a whole. Genetic selection is based upon the fact that 

 all individuals are not equally fertile, and that the type tends 

 strongly to assume that of the most prolific. Bathmic influences, 

 such as ''growth force" and orthogenetic bias, have been 

 advanced as explanations of those inherent tendencies that 

 appear to characterize most species and that give an underlying 

 trend to their direction in descent. 



The basis of all vital activities is conceived as being some 

 kind of living unit, comparable with the atom and the molecule 

 in the non-living world, whose activities constitute growth and 

 differentiation, and whose reactions with outside matter and 

 with each other are fruitful causes of variation. 



ADDITIONAL REFERENCES 



BUD VARIATION AND ITS BEARING UPON WEISMANNISM. By L. H. 

 Bailey. Science, I, 281-291. 



BUD VARIATION, CAUSES OF, AND ILLUSTRATION. By R. M. Kellog. 

 Proceedings of the Michigan Horticultural Society, 1897, pp. 121- 

 134 ; also in Experiment Station Record, XI, 424-425. 



BUD VARIATION OF CONCORD GRAPE. By W. Paddock. Garden and Forest, 

 No. 456, pp. 464-466 ; also in Experiment Station Record, VIII, 290. 



COLOR EFFECTS IN CROSSING SWEET AND FLINT CORNS. Bulletin Illi- 

 nois Experiment Station, No. 21 ; also in Experiment Station Record, 

 XIII, 740. 



DETERMINATE VARIATION AND ORGANIC SELECTION. By J. M. Baldwin. 

 Science, VI, 770-773. 



THE DEVELOPMENT OF THE HYBRIDS BETWEEN FUNDULUS HETERO- 

 CLITUS AND MENIDIA NOTATA, WITH ESPECIAL REFERENCE TO THE 

 BEHAVIOR OF THE MATERNAL AND PATERNAL CHROMATIN. By 

 W. J. Moenkhaus. American Journal of Anatomy, III, 29. 



EFFECT OF FERTILIZATION UPON FRAGRANCE. Experiment Station 

 Record, VIII, 55. 



EFFECT OF SPAYING UPON THE QUALITY OF MILK. Experiment Station 

 Record, XIV, 182. 



ESSAYS ON HEREDITY. By A. Weismann. 2 vols. 



EVOLUTION IN A DETERMINATE LINE. By Bashford Dean. Biological 

 Bulletin, VII, 105-112. 



