CLASSIFICATION. 6 1 



dition, since the descent of any given series of individuals 

 from a single pair, or from pairs exactly similar to one 

 another, is at best but a probability, and is in no case cap- 

 able of proof. In the case of the higher animals it can 

 doubtless be shown that certain assemblages of individuals 

 possess amongst themselves the power of fecundation and 

 of producing fertile progeny, and that this power does not 

 extend to the fecundation of individuals belonging to another 

 different assemblage. Amongst the higher animals, "crosses" 

 or " hybrids " can only be produced between closely-allied 

 species ; and when produced they are sterile, and are not 

 capable of reproducing their like. In these cases, there- 

 fore, we may take this as a most satisfactory element in the 

 definition of "species." The sterility, however, of hybrids 

 is not universal, even amongst the higher animals ; and 

 amongst plants no doubt can be entertained but that the 

 individuals of species universally admitted to be distinct are 

 capable of mutual fertilisation ; the hybrid progeny thus 

 produced being likewise fertile, and capable of reproducing 

 similar individuals. That this fertility is often irregular, 

 and may be destroyed in a few generations, admits of ex- 

 planation, and hardly alters the significance of these un- 

 doubted facts. 



Upon the whole, then, it seems in the meanwhile safest 

 to adopt a definition of species which implies no theory, and 

 does not include the belief that the term necessarily expresses 

 a fixed and permanent quantity. Species, therefore, may 

 be defined as an assemblage of individuals which resemble each 

 other in their essential characters, are able, directly or indirectly, 

 to produce fertile individuals, and which do not (as far as 

 human observation goes) give rise to individuals which vary 

 from the general type through more than certain definite limits. 

 The production of occasional monstrosities does not, of 

 course, invalidate this definition. 



Genus is a term applied to groups of species which pos- 

 sess a community of essential details of structure. A genus 



