350 THE BIOLOGY OF INSECTS 



naturalist's principle of organic evolution. The conception 

 of descent with modification inevitably recalls the subject 

 treated in Chapter VI. of this book, the great subject of 

 Heredity, from the study of which it is realised how through 

 the definite yet unknown factors lodged in the germ-cells, 

 characters are inherited and transmitted through successive 

 generations. Heredity has often been expounded by the 

 expression '' like begets Hke " or '' the offspring resembles 

 the parent." But the facts of inheritance, as shown by the 

 moths and fruit-flies mentioned in that chapter (pp. 121- 

 131), convince us that these expressions need qualification. 

 The likeness of offspring to parent is not absolute, members 

 of the same family may differ in various details from either 

 parent or from both as well as among themselves. Observa- 

 tion and experimental breeding among insects as among 

 other creatures, convince the student that Heredity may 

 involve Variation, and that these are not, as is often imagined, 

 antagonistic, because the variation which distinguishes a 

 member of some family from its parent or brother is due 

 to a definite factor in a germ-cell borne in that parent's 

 body. Every inherited variation that appears depends on 

 the nature of the '' physical basis of heredity " in the tgg^ 

 fertilised or unfertilised, whence the creature which shows 

 that variation has developed. Therefore, since by evolu- 

 tion we understand descent with modification, it follows 

 that in studying heredity with variation we are face to 

 face with the basal or primary factor of the process. 



The increasingly divergent groups of the systematic 

 zoologist : variety, species, genus, family, order, class (with 

 the intermediate ** sub- " and " super- " groups), suggest 

 that the wider differences, such as those of genus and 

 family, are extensions of the smaller differences such as 

 those of variety and species. Thus we are again brought 

 to the conclusion that in the observable variation, within 

 the limits of a species, which is often an accompaniment of 

 the inheritance of ancestral characters, we see, as it were, 

 the raw material for the evolutionary process. The detailed 

 and critical study of variation during the last twenty-five 



