354 FORMS OV LIFE CHANGING 



whole animal kingdom." M. Barrande has made forcible 

 remarks to precisely the same effect. It is, indeed, quite 

 futile to look to changes of currents, climate, or other phy- 

 sical conditions, as the cause of these great mutations in 

 the forms of life throughout the world, under the most 

 different climates. We must, as Barrande has remarked, 

 look to some special law. We shall see this more clearly 

 when we treat of the present distribution of organic beings, 

 and find how slight is the relation between the physical 

 conditions of various countries and the nature of their 

 inhabitants. 



This great fact of the parallel succession of the forms of 

 life throughout the world, is explicable on the theory of 

 natural selection. New species are formed by having some 

 advantage over older forms; and the forms, which are 

 already dominant, or have some advantage over the other 

 forms in their own country, give birth to the greatest 

 number of new varieties or incipient species. We have 

 distinct evidence on this head, in the plants which are 

 dominant, that is, which are commonest and most widely 

 diffused, producing the greatest number of new varieties. 

 It is also natural that the dominant, varying and far- 

 spreading species, which have already invaded, to a certain 

 extent, the territories of other species, should be those 

 which would have the best chance of spreading still further, 

 and of giving rise in new countries to other new varieties 

 and species. The process of diffusion would often be very 

 slow, depending on climatal and geographical changes, on 

 strange accidents, and on the gradual acclimatization of 

 new species to the various climates through which they 

 might have to pass, but in the course of time the domi- 

 nant forms would generally succeed in spreading and would 

 ultimately prevail. The diffusion would, it is probable, be 

 slower with the terrestrial inhabitants of distinct conti- 

 nents than with the marine inhabitants of the continuous 

 sea. We might therefore expect to find, as we do find, a 

 less strict degree of parallelism in the succession of the 

 productions of the land than with those of the sea. 



Thus, as it seems to me, the parallel, and, taken in a 

 large sense, simultaneous, succession of the same forms of 

 life throughout the world, accords well with the principle 

 of new species having been formed by dominant species 



