CHAP. IV.] NATURAL SELECTION. 91 



On the Degree to which Organisation tends to advance. 



Natural Selection acts exclusively by the preservation and 

 accumulation of variations, which are beneficial under the organic 

 and inorganic conditions to which each creature is exposed at all 

 periods of life. The ultimate result is that each creature tends to 

 become more and more improved in relation to its conditions. This 

 improvement inevitably leads to the gradual advancement of the 

 organisation of the greater number of living beings throughout the 

 world. But here we enter on a very intricate subject, for natu- 

 ralists have not defined to each other's satisfaction what is meant 

 by an advance in organisation. Amongst the vertebrata the degree 

 of intellect and an approach in structure to man clearly come into 

 play. It might be thought that the amount of change which the 

 various parts and organs pass through in their development from 

 the embryo to maturity would suffice as a standard of comparison ; 

 but there are cases, as with certain parasitic crustaceans, in which 

 several parts of the structure become less perfect, so that the 

 mature animal cannot be called higher than its larva. Von Baer's 

 standard seems the most widely applicable and the best, namely, 

 the amount of differentiation of the parts of the same organic 

 being, in the adult state as I should be inclined to add, and their 

 specialisation for different functions ; or, as Milne Edwards would 

 express it, the completeness of the division of physiological labour. 

 But we shall see how obscure this subject is if we look, for instance, 

 to fishes, amongst which some naturalists rank those as highest 

 which, like the sharks, approach nearest to amphibians; whilst 

 other naturalists rank the common bony or teleostean fishes as the 

 highest, inasmuch as they are most strictly fish-like, and differ 

 most from the other vertebrate classes. We see still more plainly 

 the obscurity of the subject by turning to plants, amongst which 

 the standard of intellect is of course quite excluded; and here 

 some botanists rank those plants as highest which have every 

 organ, as sepals, petals, stamens, and pistils, fully developed in 

 each flower ; whereas other botanists, probably with more truth, 

 look at the plants which have their several organs much modified 

 and reduced in number as the highest. 



If we take as the standard of high organisation, the amount of 

 differentiation and specialisation of the several organs in each being 

 when adult (and this will include the advancement of the brain 

 for intellectual purposes), natural selection clearly leads towards 

 this standard : for all physiologists admit that the specialisation of 

 organs, inasmuch as in this state they perform their functions 

 better, is an advantage to each being ; and hence the accumulation 

 of variations tending towards specialisation is within the scope of 

 natural selection. On the other hand, we can see, bearing in mind 



