May, 1922 common misconceptions of evolution 183 



On the other hand, a point on which the man who merely 

 reads about evolution may be at fault, is in thinking that 

 variations are always necessarily fortuitous and occur in a 

 helter-skelter fashion. No doubt many variations are of this 

 nature, but there appear to be others which are directive in 

 their nature from the beginning and which keep on increasing 

 in value with successive generations, the "rectigradations" 

 of H. F. Osborn. The observation of this sort of serial suc- 

 cessive variations has led to the suggestion of the principle 

 of orthogenesis in evolution, the idea of successive changes 

 along the same line, each going a little farther than its 

 predecessor, so that in a comparatively short time a much 

 greater distance has been compassed than would be possible 

 by mere chance variation in any or all directions. The 

 literature of paleontology is full of such examples, dealing with 

 horns, teeth, limbs, spines, shells and other structures capable 

 of fossilization. The only satisfactory explanation suggested 

 to account for this, seems to be that a small chemical change 

 in the germ plasm may make possible another change of like 

 character and this supply the basis for the next step, and so on. 

 Only on some such basis as this can we explain the evolution 

 of certain structures which make their first appearance in such 

 a small degree that they have no apparent value in selection 

 and yet they keep on varying and advancing along the same line 

 until the structure becomes either adaptive and of value 

 to the organism or inadaptive to a degree sufficient to destroy 

 the species. Such structures may sometimes rise from insig- 

 nificant, non-selective stages to a condition of much importance, 

 to the organism, but, having started to vary in one line the 

 advance may keep on beyond the adaptive condition and 

 finally become a menace to the species. Such conditions of 

 racial senescence are known in numerous examples from the 

 fossil records. 



A mistake commonly made by those not engaged in biological 

 work is to think that a great majority of the variations pro- 

 duced must have some value to the organism, since harmful 

 variations are seldom noticed in nature. It is true that 

 beneficial or at least harmless variations are the ones usually 

 noticed, because harmful variations are not perpetuated very 

 long. The biologist with an eye open to these things very 

 often observes them, but they never last long and the more 



