CHAPTERS ON EVOLUTION. 



FIG. 30. 

 BONES OK MAN'S ARM. 



exist in a rudimentary condition have been known to become func- 

 tionally active and to secrete milk; this peculiarity having been known 

 to occur even in the human subject. Amongst the higher quadrupeds, 

 however, there yet remains for extended notice one 

 special instance of the occurrence of " rudimentary 

 organs," wherein not merely is the nature of the 

 parts thoroughly determined, but the stages of their 

 degradation can be clearly traced through the 

 remarkable and fortunate discovery of the " missing 

 links." Moreover, the case in point, that of the 

 horse, so clearly illustrates what is meant by 

 progressive development or evolution of a species 

 of animals, that it is highly instructive, even if 

 regarded from the latter point of view. 



When we look at the skeleton of a horse's fore- 

 limb, we are able, without much or any previous 

 acquaintance with the facts of comparative anatomy, 

 to see that it is modelled upon a type similar to 

 that of the arm of man. Were we further to com- 

 pare the wing of the bird, the paddle of the whale, 

 the fore-limb of the bat, and the fore -leg of a lizard or frog, with 

 the equine limb, we should find the same fundamental type of 

 structure to be represented in all. Thus we 

 find in the arm of man (Fig. 30) to select 

 the most familiar example from the series 

 just mentioned a single bone, the 

 humerus ( 3 ), forming the upper arm: two 

 bones (radius [ 4 ] and ulna [ 8 ] ) constituting 

 the fore-arm : eight small bones forming the 

 wrist (carpus) : five bones one for each 

 finger forming the palm or metacarpus : 

 and five fingers, each composed of three 

 small bones, named phalanges, with the 

 exception of the thumb, in which, by a mere 

 inspection of that digit, we may satisfy our- 

 selves only two joints exist In the wing 

 of the bird (Fig. 31) we similarly find an 

 upper-arm bone or humerus (a) : two bones 

 (radius [c\ and ulna \d~\ ) in the fore-arm : a 

 FIG. 31. wrist (b) : a thumb ( g) : and two fingers 



BONES OF BIRD'S WING. //"//{ 



Now, turning to the fore-limb of a horse (Fig. 32) the hind limb 

 being essentially similar in its general conformation, and correspond- 

 ing as closely with man's lower limb we find its conformation to 

 correspond in a remarkable fashion to that of man's arm. First, 



