EVOLUTION AND NATURAL SELECTION 5 



the interpretations of naturalists, but the most satisfactory one 

 is as follows: "A group of individuals which, however many 

 characters they share with other individuals, agree in present- 

 ing one or more characters of a pecuUar and hereditary kind 

 with some certain degree of distinctness." Or another defi- 

 nition of a species may be: A group of individuals bearing 

 close resemblance and primary relationships to each other. 

 These resemblances include similarities even in minute struc- 

 ture, yet varying and occasionally mutating. 



Those species living to-day are the latest surviving end 

 product in the evolution of organic life. Individuals of a 

 species interbreed freely. It has been held that one of the 

 distinguishing points of species is that when they are crossed 

 the resulting hybrid offspring are not fertile, but experimen- 

 tation and observation have shown that in plants and animals 

 this test of species does not hold good. Organisms are sensi- 

 tive in their organization and respond to their environment, 

 and this response to the surrounding influence is supposed to 

 set up a tendency to vary. No two individual animals or 

 plants are exactly alike. Systematic species as determined 

 by eminent authorities are as a rule compound groups, often 

 made up of two or three elementary types, and in some cases 

 they comprise numerous constant and well-defined forms. 

 The subdivisions of a species are distinguished from each 

 other by more than one character, often by slight differences 

 in nearly all their organs and qualities, and are designated 

 elementary species. It is very diflacult to estimate systematic 

 differences on the ground of comparative studies alone, on 

 account of the great number of varieties of the supposed 

 type. The type from which a new species is described is to 

 a certain extent only tentative, the final judgment in regard 

 to it being more exactly determined only after direct experi- 

 mental breeding. A mutation is usually rare and is often 

 confined to one individual, while varieties are numerous in 

 individuals. The difference exhibited by a sport may amount 

 to a sudden, well-marked change, and present some modi- 

 fication strictly different from the parent type. A genus 

 is an assemblage of species presenting many fundamental 

 characters in common. Some genera are clearly definable, 



