354 THE INDUCTIONS OF BIOLOGY. 



two orders of units have had their unlikenesses so far dimin- 

 ished that they will no longer do this. The same principles 

 explain for us the variable results of union between nearly- 

 related organisms. According to the contrasts among the 

 physiological units they inherit from parents and ancestors; 

 according to the unlike proportions of the contrasted units 

 which they severally inherit; and according to the degrees 

 of segregation of such units in different sperm-cells and 

 germ-cells; it may happen that two kindred individuals will 

 produce the ordinary number of offspring or will produce 

 none; or will at one time be fertile and at another not; or 

 will at one time have offspring of tolerable strength and at 

 another time feeble offspring. 



To the like causes are also ascribable the phenomena of 

 Variation. These are unobtrusive while the tolerably-uni- 

 form conditions of a species maintain tolerable uniformity 

 among the physiological units of its members; but they be- 

 come obtrusive when differences of conditiohs, entailing con- 

 siderable functional differences, have entailed decided differ- 

 ences among the physiological units, and when the different 

 physiological units, differently mingled in every individual, 

 come to be variously segregated and variously combined. 



Did space permit, it might be shown that this hypothesis 

 is a key to many further facts to the fact that mixed races 

 are comparatively plastic under new conditions; to the fact 

 that pure races show predominant influences in the offspring 

 when crossed with mixed races ; to the fact that while mixed 

 breeds are often of larger growth, pure breeds are the more 

 hardy have functions less-easily thrown out of balance. 

 But without further argument it will, I think, be admitted 

 that the power of this hypothesis to explain so many pheno- 

 mena, and to bring under a common bond phenomena which 

 seem so little allied, is strong evidence of its truth. And 

 such evidence gains greatly in strength on observing that 

 this hypothesis brings the facts of Genesis, Heredity, and 

 Variation into harmony with first principles. We see that 



