xiii EXTENSIONS OF DARWINISM 257 



vascular system, arterial and venous, would have to undergo succes- 

 sive unbuildings and rebuildings to make its channels everywhere 

 adequate to the local requirements, since any want of adjustment 

 in the blood-supply to this or that set of muscles would entail in- 

 capacity, failure of speed, and loss of life. Moreover, the nerves 

 supplying the various sets of muscles would have to be appropriately 

 changed, as well as the central nervous tracts from which they 

 issued. Can we suppose that all these appropriate changes, too, 

 would be, step by step, simultaneously made by fortunate spon- 

 taneous variations occurring along with all the other fortunate 

 spontaneous variations? Considering how immense must be the 

 number of these required changes, added to the changes above 

 enumerated, the chances against any adequate readjustments for- 

 tuitously arising must be infinity to one." 



Now, this seems very forcible, and has, no doubt, con- 

 vinced many readers. Yet the argument is entirely fallacious, 

 because it is founded on the tacit assumption that the number 

 of the varying individuals is very small, and that the amount 

 of coincident variation is also both small and rare. It is 

 further founded on the assumption that the time allowed for 

 the production of any sufficient change to be of use is also 

 small. But I have shown in the early chapters of this book 

 (and much more fully in my Darwinism) that all these 

 assumptions are the very reverse of the known facts. The 

 numbers of varying individuals in any dominant species (and 

 it is only these which become modified into new species) is 

 to be counted by millions ; and as the whole number can, as 

 regards any needed modification, be divided into two halves 

 — those which possess the special quality required above or 

 below the average — it may be said that nearly half the total 

 number vary favourably, and about one-fourth of the whole 

 number in a very large degree. Again, it has been shown 

 that the number of coincident variations are very great, 

 since they are always present when only a dozen or twenty 

 individuals are compared ; but nature deals with thousands 

 and millions of individuals. Yet, again, we know that 

 changes of the environment are always very slow as measured 

 by years or generations, since not a single new species is 

 known to have come into existence during the whole of the 

 Pleistocene period ; and as fresh variations occur in every 

 generation, almost any character, with all its co-ordinated 



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