ON THE CAUSES OF VARIATION. 819 



To the imperfection of the geologic record is to he attributed, no 

 doubt, a large number of these gaps yet existing between types, 

 and many important links or branches are yet to be discovered. 

 Yet the views we have been considering should absolve evolution- 

 ists from all necessity of demonstrating the more minute grada- 

 tions ; because, in deposits like the Tertiary, during which we 

 may assume life-conditions to have remained comparatively uni- 

 form, these saltations take place. Saltation, or, what is probably 

 a truer expression, wave-movement, would indeed seem to be a 

 prerequisite of progress, and will account for much that is going 

 on even at the present day. In artificial selection by man we find 

 that it is at first comparatively easy to accumulate minute pecul- 

 iarities and variations by rigid breeding and exclusion of all devi- 

 ation ; but that we soon arrive at a fixed point which is maintained 

 at first with difficulty but with increasing ease with each genera- 

 tion. During these more fixed periods the potentiality for change 

 is doubtless increasing, until at last it is suddenly manifested in 

 renewed variation. Rest is followed by activity just as surely 

 as activity induces and requires rest. 



There is a limit to development in organs, just as there is a 

 limit to individual mental growth. Weariness of previous effort 

 comes upon us when the limit of result is attained, accompanied 

 by great longing for change, and not infrequently with revulsion 

 from previous effort. The naturalist who has devoted a part of 

 his life to the persistent accumulation of facts and specimens, has 

 held the imaginative and generalizing powers in abeyance during 

 that period. The reserve brain -force in this direction may be 

 suddenly called into activity by exhaustion in the other, and the 

 process may perhaps be comparable to the exhaustion of the soil 

 for one particular crop, without lessening its fertility for some 

 other, the recognition of which fact is the foundation of all suc- 

 cessful agriculture. Excess of development, whether in body or 

 mind, inevitably brings about either wholesome reaction or utter 

 collapse. 



How far the rhythmic tendency in the development of animal 

 life may be explained by the rapid change of climate, by migration 

 and the loss of record, or upon the general law that while there 

 has been progress of the whole, there has not necessarily been 

 progress of every part, it would take us too far to discuss in this 

 connection. I think we are safe in saying, however, that the 

 facts justify the belief that, in the evolution of animal life, as in 

 the evolution of everything else, jjrogress has often been made 

 by waves. 



FisJce's Laiv. — With regard to what may be called Fiske's 

 law of correlation between brain development and infantile de- 

 pendency, Fiske has so admirably elaborated the subject that it 



