104 FORMS OF LIFE CHANGING [Chap. XL 



new varieties. It is also natural tliat tlie dominant, 

 varying, and far-spreading species, which have akeady 

 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 geograpliical changes, on strange acci- 

 dents, and on the gradual acclimatisation of new species 

 to the various climates through which they might have to 

 pass, but in the course of time the dominant forms would 

 generally succeed in spreading and would ultimately 

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

 with the terrestrial inhabitants of distinct continents 

 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 pro- 

 ductions 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 domi- 

 nant species spreading widely and varying ; the new 

 species thus produced being themselves dominant, 

 owing to their having had some advantage over their 

 already dominant parents, as well as over other species, 

 and again spreading, varying, and producing new forms. 

 The old forms which are beaten and which yield their 

 places to the new and victorious forms, will generally 

 be allied in groups, from inheriting some inferiority in 

 common ; and therefore, as new and improved groups 

 spread throughout the world, old groups disappear from 

 the world; and the succession of forms everywhere 



