430 ROSS ON EVOLUTION. 



20, Devonian 24, Carboniferous 30, Triassic 35, Jurassic 49, 

 Cretaceous 56, Tertiary 62. The Genera for the same Formations 

 in the same Order were 53, 77, 79, 81, 108, 148, and 192 respec- 

 tively. The decrease of such Families as have become extinct, or 

 seem in process of extinction, is a similarly gradual process, and 

 occurs first in the Genera, least typical of the Family, or most synthe- 

 tic in type ; so that both increase and diminution seem to follow an 

 organic law, which may be illustrated (though of course the analogy 

 is far indeed from being perfect) by the growth of a branch of say 

 a fir tree, and in the case of the Families which have become 

 extinct the gradual withering and successive death of the branch lets, 

 until finally the topmost bud, and with it of course, the branches 

 succumbed to the crowding and pressure of the superior and sur- 

 rounding branches. Of course the regularity of this process in the 

 family is interrupted by the fact that in isolated areas the older 

 types may be preserved and even extended. 



Another difficulty in defining the limits of Species arises from 

 the fact of the intercrossing of Species of the same Genus, and 

 although the product is usually sterile except with either of the 

 original Species, yet the incorporation by this means of an element 

 from one Species into another, seems incompatible with the idea of 

 the two Species having been originally distinct and without any 

 generic affinity ; but the difficulty of entertaining such a supposition 

 becomes still more striking when the product of such intercrossing 

 of Species is fertile inter se, as in a case described by the late Prof. 

 Agasslz, and where it is evident that the new Species (shall I say 

 since there Is no other possible way of classifying it) might continue 

 to exist, In Its entirely distinct form, throughout a Geological Epoch, 

 if brought to a South Pacific Island, where it would be alike free 

 from competition and from admixture with allied Species ; indeed In 

 the case referred to below it seems likely to be continued as such in 

 a domestic condition for economic reasons. Agasslz says : " There 

 are, however, two animals entirely distinct as to specific characters 

 — the hare and the rabbit of Europe ; (I do not speak of those of 

 the United States, respecting which such observations have not yet 

 been made) : these animals have been crossed and ofl^spring has 

 proved to be fertile, not only with the original Species, the hare 



