WHAT ARE SPECIES/ 411 



gradation can be traced between those forms which have the pecu- 

 liarity strongly marked and those in which it is absent. 



Tims far the considerations which guide the biologist in the estab- 

 lishment of species differ in no respect from those which influence the 

 mineralogist. 



But although naturalists have no more direct knowledge of any but 

 the morphological character of the great majority of the species of 

 animals and plants than they would have of so many mineral speci- 

 mens, they are familiar with many animals and plants in the living 

 state when they exhibit phenomena to which the mineral world pre- 

 sents no parallel, and the study of these phenomena of active life has 

 complicated the conception of species in biology, by adding physio- 

 logical to morphological considerations. 



The fact that living beings originate by generation from other 

 living beings is one of the circumstances in their history which most 

 completely differentiates them from minerals. This process of gener- 

 ation enters in various ways into the conception of biological species. 



For example, it is a generally assumed axiom in biology that 

 whatever proceeds from a living being by way of generation is of 

 the same species as that from which it proceeds, whether the morpho- 

 logical differences between parent and offspring be great or small. 

 The two sexes are often extraordinarily different, and in cases of the 

 so-called alternation of generation the successive zooids may differ 

 very widely ; but, inasmuch as the differing forms in these cases pro- 

 ceed from the same parents, no one doubts that they belong to the same 

 species. The breeds of domesticated animals and plants often differ 

 morphologically as widely as admitted species, but, apart from other 

 considerations, historical evidence that they have the same parentage 

 suffices to cause them to be regarded as of one species. It is not 

 quite clear that the converse of the axiom which has just been re- 

 ferred to would be admitted, and that living beings which arise from 

 totally distinct parents are of different species, even though morpho- 

 logically identical. The wellnigh exploded hypothesis of the multi- 

 plicity of centres of origin for species of wide distribution implies the 

 belief that groups of individuals which have proceeded from distinct- 

 ly-created parents may, nevertheless, be of the same species, while 

 the supporters of the no less nearly extinct hypothesis of the inde- 

 pendent creation of the fauna and flora of successive formations used 

 to affirm that, although indistinguishable, two forms from separate 

 formations must be of distinct species, because they had been created 

 separately. However, these subtilties have ceased to have any prac- 

 tical importance. 



In the next place it is observed that, while individuals of the same 

 morphological species breed freely with one another and give rise to 

 perfectly fertile offspring, the unions of individuals of different mor- 

 phological species are, as a rule, either unfertile or imperfectly fertile. 



