EFFECTS OF NATURAL SELECTION 13S 



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 num- 

 ber of living beings throughout the world. But here we enter 

 on a very intricate subject, for naturalists have not defined 

 to each other's satisfaction what is meant by an advance in 

 organization. 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 dif- 

 ferent 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 in- 

 stance, to fishes, amongst which some naturalists rank those 

 as highest which, like the sharks, approach nearest to amphi- 

 bians; whilst other naturalists range 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 advance- 

 ment of the brain for intellectual purposes), natural selec- 

 tion clearly leads towards this standard: for all physiologists 

 admit that the specialisation of organs, inasmuch as in this 



