20 MANUAL OF ZOOLOGY. 



three entirely different forms, not connected by any interme- 

 diate links, and the same thing occurs in some of the Crustacea. 



As regards, therefore, the first point in the definition of 

 species, namely the external resemblance of assemblages of 

 individuals, we are forced to conclude that no two individuals 

 are exactly alike ; and that the amount and kind of external 

 resemblance which constitutes a species is not a precise and 

 invariable quantity, but depends upon the value attached to 

 particular characters by any given observer. 



The second point in the definition of species, namely com- 

 munity of descent, is hardly in a more satisfactory condition, 

 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 capable 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 in- 

 dividuals belonging to another different assemblage. Amongst 

 the higher animals ' crosses ' or ' hybrids ' can only be pro- 

 duced between closely allied species, and, when produced, 

 they are sterile, and are not capable of reproducing their like. 

 In these cases, therefore, we may take this as a most satis- 

 factory 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 

 rnay be destroyed in a few generations, admits of explanation, 

 and hardly alters the significance of these undoubted 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 pro- 

 duce fertile individuals, and which do not (as far as human ob- 

 servation 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 possess 

 a community of essential details of structure. A genus may 

 include a single species only, in cases where the combination 



