CHAPTER VII 

 HEREDITY 



By " heredity " we mean that the progeny of an animal belongs 

 to the same category of organisms as do its parents. This 

 definition is of the nature of a '' first approximation " and it will 

 be amplified in the following pages. 



77. ON THE CATEGORIES OF ANIMALS 



Investigations of the structures of living and fossil animals 

 enable us to make a hierarchy of categories : it is necessary, first 

 of all, to base these categories on structural characters so that 

 we may include fossil forms of which we have only (partial) 

 knowledge of structure. Afterwards we may base the categories, 

 so far as possible, upon the habits and life-histories of living 

 animals. The categories are logical constructions and they are 

 made by naturalists rather than being " in nature." What we 

 observe in nature are individual organisms and we classify these. 

 Thus while the broad outlines of classifications are generally 

 agreed upon by systematists there is much diflFerence of opinion 

 upon the finer details and these divergences represent not only 

 imperfect inductions but also different criteria as to the formula- 

 tion of the categories. 



A classification is hierarchical. At its base are species. Groups 

 of species that have certain characters in common are called 

 Genera ; groups of genera are families ; groups of families are 

 Orders ; groups of orders are Classes ; groups of classes are 

 Phyla and a small number of Phyla constitute the animal kingdom. 

 But even upon the formulation of the hierarchy there are 

 differences of outlook. All these categories, sub -categories, etc., 

 are plainly logically constructed concepts, but something more 

 must be said about species. 



77«. Species. These are, to some extent, natural categories 

 in that they are '' in nature." The individual animals that 



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