xni HEREDITY AND VARIABILITY 147 



or the evolution of the individual, is, in its main features, a 

 recapitulation of phytogeny or the evolution of the race. 



One other matter must be referred to in concluding the 

 present lesson. It is obvious that the evolution of one 

 species from another presupposes the occurrence of varia- 

 tions in the ancestral form. As a matter of fact such 

 individual variation is of universal occurrence : it is a matter 

 of common observation that no two leaves, shells, or human 

 beings are precisely alike, and in our type genus Zootham- 

 nium the number of zooids, their precise arrangement, the 

 details of branching, &c., are all variables. This may be 

 expressed by saying that heredity, according to which the 

 offspring tends to resemble the parent in essentials, is 

 modified by variability, according to which the offspring 

 tends to differ from the parent in details. If from any 

 cause an individual variation is perpetuated there is produced 

 what is known as a variety of the species, and, according to 

 the theory of the origin of species by evolution, such a 

 variety may in course of time become a new species. Thus 

 a variety is an incipient species, and a species is a (relatively) 

 permanent variety. 



It does not come within the scope of the present work to 

 discuss either the causes of variability or those which deter- 

 mine the elevation of a variety to the rank of a species : 

 both questions are far too complex to be adequately treated 

 except at considerable length, and anything of the nature of 

 a brief abstract could only be misleading. As a preliminary 

 to the study of Darwin's Origin of Species, the student is 

 recommended to read Romanes's Evidences of Organic 

 Evolution, in which the doctrine of Descent is expounded 

 as briefly as is consistent with clearness and accuracy. 



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