i 4 THE GROWTH OF GROUPS 



to nature. The potato beetle, Lcptinotarsa rubicunda, has 

 appeared as the offspring of the commoner L. multitaniata 

 and hence is derived from that species. But it is " almost 

 completely " sterile with the parent species though 

 perfectly fertile in itself. It is therefore almost com- 

 pletely a species in the terms of the first definition and 

 entirely a variety in terms of the second. 



Let us consider some other definitions. Species are 

 groups which differ from one another in all their charac- 

 teristics, whereas varieties differ from one another or 

 from the parent species in only one or two well-marked 

 characters. If we employ these definitions, we may often 

 be sure that this group is a species and that a variety ; 

 but frequently we shall be in difficulty, since there are 

 many groups in the animal kingdom which appear to 

 have two, three, or more characters of their own, but yet 

 do not differ from allied groups in all respects. Such 

 groups are common among the higher animals, and 

 although they are neither species nor varieties in the 

 terms of the last-mentioned definitions, yet they are 

 always regarded as species by naturalists. It is necessary 

 to define the term group as used in this book, and in doing 

 so a certain conception will be employed which has 

 found wide but not perhaps general approval. Mainly 

 as a result of Mendel's researches, we perceive that 

 organisms contain certain attributes, which are distinct 

 in their relation to the rest of the organism. These 

 attributes or characters appear as separate entities, 

 since each as a unit may be present in the body or absent 

 from it without affecting the remainder of the body. This 

 is assumed here to be true without discussing the physio- 

 logical proof, the segregation of characters in the germ cells. 



