STATISTICAL BIOLOGICAL STUDY. 39 



A basis for an arbitrary distinction between species 



and varieties may be gained by determining a degree of 

 divergence and of isolation which shall be used to distinguish 

 the two. A degree of divergence of thrice the standard devia- 

 tion has been suggested as a convenient line between species 

 and varieties. 



Quantitative studies in correlation will give us new cri- 

 teria for homology by telling us the relative morphoge- 

 netic kinship of the parts of the body. 



Quantitative studies in heredity will give definitive informa- 

 tion on prepotency of sex or race. By examining hybrids 

 quantitatively and comparing them with their parents we shall 

 unravel the laws of inheritance in cross-breeding and the prin- 

 ciples of mixing characters in biparental inheritance. 



In a word, by the use of the quantitative method biology 

 will pass from the field of the speculative sciences to that of 

 the exact sciences. 



