J-S6 EXPERIMENTAL STUDY OF INHERITANCE 



and black grouse, carrion crow and hooded ciow, different 

 species of Saiurnia, different species of Medicago. 



Individuals belonging to different subspecies — e.g. maize. 



Individuals belonging to different breeds — e.g. poultry, 

 short-horn and Aberdeenshire Angus cattle, Clydesdale and Shire 

 horses, silkmoths. 



Individuals belonging to different " varieties " which have 

 not risen to the stability of " breeds " — e.g. wheat susceptible 

 and immune to rust. 



Hybridisation of Distinct Species. — The conception of species 

 is confessedly quite relative — it is a term of convenience when we 

 wish to include under one title all the members of a group of 

 individuals who resemble one another in certain characteristics. 

 A species is often simply a segment of a curve of closely related 

 forms. It is a statistical conception, and as there is no abso- 

 lute constancy in specific characters, as one species melts into 

 another, with which it is connected by intermediate varieties, 

 by frequent or casual variations, we have to confess that it is 

 a human device, the validity of which varies greatly according 

 to our knowledge or ignorance of the forms in question. A specific 

 name is sometimes, when we are very ignorant, as unmeaning as 

 the name of a constellation in the starry heavens. But it is 

 equally convenient. 



At the same time, since science is systematised common sense, 

 it is usually admitted — oftener, perhaps, as a pious opinion, than 

 as a practice — that the characters on account of which a naturalist 

 gives a specific name to a group of similar individuals should he 

 more marked than those which distinguish the members of any one 

 family, should show a relative constancy from generation to genera- 

 tion, and shotdd be associated with reproductive pectdiarities which 

 tend to restrict the range of mutual fertility to the members of the 

 proposed species (see the author's Outlines of Zoology, 4th ed., 

 1906, pp. 14-16). 



The popular impression that crosses between " distinct 



