306 THE ORIGIN OF SPECIES 



invaded, to a certain extent, the territories of other species, should 

 be those which would have the best chance of spreading still far- 

 ther, 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 ac- 

 cidents, 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 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 de- 

 gree 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 through- 

 out the world, accords well with the principle of new species hav- 

 ing been formed by dominant species spreading widely and vary- 

 ing; 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 spread- 

 ing, 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 ever3^where tends to cor- 

 respond both in their first appearance and final disappearance. 



There is one other remark connected with this subject worth 

 making. I have given my reasons for believing that most of our 

 great formations, rich in fossils, were deposited during periods of 

 subsidence; and that blank intervals of vast duration, as far as 

 fossils are concerned, occurred during the periods when the bed 

 of the sea was either stationary or rising, and likewise when sedi- 

 ment was not thrown down quickly enough to embed and preserve 

 organic remains. During these long and blank intervals I suppose 

 that the inhabitants of each region underwent a considerable 

 amount of modification and extinction, and that there was much 

 migration from other parts of the world. As we have reason to 

 believe that large areas are affected by the same movement, it is 

 probable that strictly contemporaneous formations have often 

 been accumulated over very wide spaces in the same quarter of 

 the world ; but we are very far from having any right to conclude 



