SPECIES. UNITY. 21 



obstructions prevented their migration from one region to 

 another. 



Thus, Agassiz and others cite the Asiatic and African lion ;* 

 Vogt, the chamois of the Pyrenees and of the Alps ; the mouflon 

 in Sardinia and Asia Minor, which though they differ very 

 little from each other, cannot be considered as belonging to the 

 same stock, and are consequently not of the same species. 

 Giebel, 2 especially, has quoted a large number of examples 

 which seem to prove that the assumption of single prototypes 

 for individual species of animals, is in many cases untenable, 

 partly because an existence in masses is in many cases requi- 

 site for the nutriment of others, partly because the migratory 

 capacity of many is too limited to admit of their gradual pro- 

 pagation over the whole regions which they at present occupy ; 

 such is the case as regards the mole, the beaver, many snails, 

 and most fresh- water animals. Gregarious animals can scarcely 

 be considered as having descended from a single pair. Hence, 

 several centres of creation have been assumed, at least for some 

 genera. Here it is especially necessary to distinguish unity of 

 descent from unity of species. This may, perhaps, be done by 

 assuming, whenever the facts require it, that a species consists 

 of ' ' homogenous species/' ' ' sub-parallel species," or " stocks," 

 namely, where in individuals of an ascertained or strongly pre- 

 sumptive different stock, the usual limits of variation within 

 the same stock are not passed, and the physical and mental 

 development is essentially the same ; so that according to Pri- 

 chard's expression, " there is nothing in the way to consider 

 them as the descendants of the same stock ." And when he 

 further agrees with De Candolle, that it does not unfrequently 

 occur that two individuals who really belong to the same spe- 

 cies, whose common descent is incontestable, yet differ more in 

 their external aspect than others of a different species, we are 

 reminded of the uncertainty of all conclusions as regards unity 

 of stock if inferred alone from similarity of type. 



1 Swainson (" Treatise on the Geography and Classification of Animals/' 

 p. 284, 1835) is inclined to assume five distinct species of lions, and quotes 

 other similar instances. 



8 Tagesfragen aus d. Naturgesch.," p. 69, 1857. 



