OTHER THEORIES OF EVOLUTION 323 



that there are usually large gaps into which the excess popula- 

 tion of a species may spread are uncertain. 1 1 is certainly known 

 that a number of intentionally introduced insects have failed 

 to establish themselves. But it seems likely that opportunities 

 for spreading in this manner may not be uncommon. 



The researches of Gulick, Crampton and other workers on 

 the local and racial divergence of land snails and the studies 

 of various workers on the local diversification of mammals, 

 birds, reptiles and fishes have all tended to show that a very 

 substantial amount of subspecific and specific divergence may 

 arise in conditions in which selection may with all likelihood 

 be excluded. Studies such as those of Crampton make it 

 almost certain that local divergence is established in conditions 

 in which neither the effect of the environment nor adaptation 

 to local conditions is to be held responsible. But it is one 

 thing to show that under isolation certain recombinations of 

 characters may be maintained as separate entities or even that 

 entirely new mutations may be established, and another to 

 show how such divergences may be amplified until they give 

 rise to marked and sustained evolutionary series. In short, 

 while it is likely that local races have arisen without the aid of 

 selection, we do not see how such divergences could have been 

 continuously amplified without some directive process. 



Knowing as little as we do about mutation-rates in nature 

 it is useless to indulge in speculations in which these rates are 

 involved. Though there is a theoretical possibility that some- 

 times a given mutation might turn up very frequently, such 

 ' mass-mutation ' is not likely to be ample enough to transform 

 whole populations. 



In conclusion, it seems to us that some measure of local 

 diversification within a species may arise in one or another of 

 the ways just indicated. We do not, however, believe that this 

 accounts for the main evolutionary tendencies. 



IV. Orthogenesis. — Various dissimilar phenomena have 

 been described under this name and some confusion has arisen 

 as to the correct use of the term. A clear account of the various 

 uses to which the term has been put and of the various concepts 

 involved is given by Kellogg (1907, p. 275 and foil.). We 

 confine our historical account to a brief recital of the essentials 

 and some additions to Kellogg's statement. 



The term was first introduced by Haacke (1897) and was 



