go SECTIONAL ADDRESSES 



tion will continue to push the stock further and further along the line of 

 development until a limit of perfection has been reached. 



This limit is usually determined by quite simple mechanical principles. 

 A horse cannot reduce its digits below one per foot, nor can it complicate 

 the grinding surface of its molars beyond a certain point without making 

 the grinding ridges too small for the food to be ground. The selective 

 advantages of mere size, which must often be great in early stages of a 

 trend, will be later offset by reduction of speed, or difficulties of securing 

 sufficient food, or, in land animals, by the relative increase of skeleton. 

 There is a limit to the acuity of vision, the streamlining of aquatic form, 

 or the length of a browser's neck, which can be useful. When these 

 biomechanical limits have been reached, the trend ceases, and the stock, 

 if it is not extinguished through the increasing competition of other 

 types, is merely held by selection to the point it has reached. 



The only feature inviting orthogenetic explanation is the directive 

 character of the trends, their apparent persistence towards a predeter- 

 mined goal. But on reflection this too is seen to be not only explicable 

 but expected on a selectionist viewpoint. Once a trend has begun, 

 much greater changes will be necessary to switch the stock over to some 

 other mode of life than to improve the arrangements for the existing 

 mode of life ; and the further a specialised trend has proceeded, the deeper 

 will be the groove in which it has thus entrenched itself. Specialisation, 

 in so far as it is a product of natural selection, automatically protects 

 itself against the likelihood of any change save further change in the same 

 direction. 



However, that this apparent orthogenesis is determined functionally 

 is excellently shown by the evolution of the elephants. These began 

 their career by an elongation of the muzzle involving the enlargement of 

 both jaws and both upper and lower incisor tusks. Before the beginning 

 i of the Pliocene, this process had reached what appears to have been a 

 mechanical limit. In the later evolution of the stock the jaws were 

 shortened, the trunk elongated, and the lower tusks abolished. The 

 effective reach of the animal for its food was continuously increased ; 

 but the structural basis was wholly altered. It is impossible to stretch 

 the principle of internal orthogenesis to cover a process of this type. 



While on this subject, we may deal with a cognate point, the so-called 

 law of the irreversibility of evolution. This is an empirical fact of pate- 

 ontology, but that it involves no intrinsic necessity is shown by the 

 experimental findings of Sewall Wright on guinea-pigs. He was able 

 to build up a stock which was in full possession of the hind little toe 

 that the wild species genus, and family, had definitively lost. Thus 

 Nature no more abhors reverse evolution than she abhors a vacuum. 



The same principles would seem to apply in general to small-scale 

 adaptations as to long-range adaptive trends, except that since such 

 adaptations frequently concern only one particular function and not the 

 organism's main way of life, it should be easier for evolutionary direction 

 to be changed, and for adaptation to set off on a new tack. 



An important difference will be found between abundant and scarce 

 species. In the latter, competition will be more with other species, 

 while in the former it will be more between members of the species 



