ROSS ON EVOLUTION. 431 



and tlie rabbit, but the cross breeds themselves, the individuals 

 derived from the crossing of hare and rabbit have been fertile 

 among themselves. Thus a new breed, which thus far exists only 

 in domesticity, has been produced and is known under the name of 

 leporide in the Paris market, where it is as common now as the 

 hare or rabbit. This new breed differs in the colour of the flesh 

 from both hare and rabbit, the former being dark the latter white, 

 while the leporide has an intermediate condition of meat much 

 esteemed for its flavor and delicacy." 



The total number of known distinct existing Species (so 

 called) of Animals and Plants is about half a million. Of these 

 it may be said that the higher the grade to which they belon*?, and 

 the higher the group within that grade, the shorter lived or lees 

 persistent is the Species, and the wider the limits of its variation, 

 so much so that in the case of many it is difficult or impossible to 

 decide as to the limits of each. Indeed every attempt to define 

 absolutely what constitutes a Specific distinction, has resulted in 

 failure, and we are left to accept the opinion of Agassiz, that a 

 Species is an ideal " entity," in no way different in kind, but only 

 in degree, from Genera, Family, Order, &c. A hundred illustra- 

 tions might be given of the difficulty, or rather the impossibility of 

 determining absolutely whether certain groups should be considered 

 as constituting a Genera consisting of a number of Species, or a 

 Species consisting of a number of Varieties. I avail myself of the 

 case of JRuhicSf so well elaborated by Prof. Lawson, and doubtless 

 fresh in the recollection of members. (See Trans, pages 364—6.) 

 *' This is particularly the case with regard to the European JRuhi 

 fruticosi, many of the long recognised species of which are so 

 closely related, that some of our best botanists now rank upwards 

 of twenty forms that are too well marked and too constant to be 

 mere varieties, as so many sub-species under the specific type of 

 Huhus fruticosus,^^ 



In estimating then the total number of existing Species, the 

 impossibility of defining the limits of each Species is in itself an 

 insuperable barrier to complete success. It must also be remem- 

 bered that while the land surface of the Earth, and the shallow seas 

 are far from having been completely explored, the deep sea forms 



