INDEFINITELY FROM THE ORIGINAL TYPE. 37 



main preponderant in numbers, and under adverse 

 physical conditions again alone survive. But this 

 new, improved, and populous race might itself, in 

 course of time, give rise to new varieties, exhibiting 

 several diverging modifications of form, any of which, 

 tending to increase the facilities for preserving ex- 

 istence, must, by the same general law, in their 

 turn become predominant. Here, then, we have 

 progression and continued divergence deduced from the 

 general laws which regulate the existence of animals 

 in a state of nature, and from the undisputed fact 

 that varieties do frequently occur. It is not, how- 

 ever, contended that this result would be invariable; 

 a change of physical conditions in the district might 

 at times materially modify it, rendering the race 

 which had been the most capable of supporting ex- 

 istence under the former conditions now the least so, 

 and even causing the extinction of the newer and, 

 for a time, superior race, while the old or parent 

 species and its first inferior varieties continued to 

 flourish. Variations in unimportant parts might 

 also occur, having no perceptible effect on the life- 

 preserving powers ; and the varieties so furnished 

 might run a course parallel with the parent species, 

 either giving rise to further variations or returning 

 to the former type. All we argue for is, that cer- 

 tain varieties have a tendency to maintain their 

 existence longer than the original species, and this 

 tendency must make itself felt ; for though the doc- 

 trine of chances or averages can never be trusted to 



