14 COMPARATIVE ANATOMY AND PHYSIOLOGY. 



All the steps in the differentiation and elaboration 

 of organised beings are examples of that process of 

 evolution, which, when based on a belief in the 

 existence of a blood relationship between animals, is 

 known as the doctrine of descent. In an attempt to 

 understand how this has worked, we make reference to 

 two different series of facts. We have, in the first 

 place, to make certain generalisations as to the way in 

 which the differences have been brought about, and we 

 have, in the second, to consider what are the essential 

 properties of living matter which may be regarded as 

 the determining factors in the evolution of organised 

 material. 



The generalisations made from a number of ob* 

 served facts may, if this definition be borne in mind, 

 be called the laws of evolution; they have been thus 

 enunciated by Professor Huxley : 



(1) There has been an excess of development of 

 some parts in relation to others. 



(2) Certain parts have undergone complete or 

 partial suppression. 



(3) Certain parts, which were originally distinct, 

 have coalesced. 



Let us apply these laws to a concrete example, and 

 select for study the fore-foot of a camel. In the more 

 primitive mammalia there were five fingers or digits, 

 each connected by a metacarpal or palm bone with 

 the wrist, and these five sets of digits and metacarpals 

 were of subequal size. In the hoofed group of animals 

 the first of these, or thumb, disappeared, as in the case of 

 the modern pig ; the two that were now outermost, the 

 second and fifth, became smaller and smaller, as in the 

 sheep or deer, and finally, as in the camel, disappeared 

 altogether. Here we have various stages of law 2. 

 This loss of the outer was accompanied by an increase 

 in the size of the median digits and metacarpals 

 (law 1), and in the more or less complete fusion of 



