82 SCIENCE PROGRESS 



and stones. Is it conceivable that they will ever lose faith in 

 their teeth and take to either sticks or stones ? The criterion 

 of success in the struggle for existence is survival, and therefore 

 unsuccessful experimenters always die out. Hence the advance 

 of mammalian life is clearly the triumph of specialisation — the 

 generalised forms in the Eocene were all small animals. When 

 once a species is committed to any line of evolution — special 

 teeth or special legs — it must make a success along that line 

 or else it will become extinct. It cannot retrace its steps and 

 begin again. Hence Dollo, of Brussels, has propounded his law 

 of the irreversibility of the course of evolution. 



The one outstanding feature in the case of man is his brain. 

 On the theory that man separated from the chimpanzee and 

 gorilla when they were about the size of the smaller of these 

 apes, we are faced with this problem. Man has made little 

 addition to the body-weight of the common ancestor, while his 

 brain has increased threefold and the canines and bones of 

 the face have been reduced in size. No cause can be assigned 

 for the overgrowth of the brain. In the parallel history of 

 the horse the disappearance of the lateral digits has been corre- 

 lated with life on the hard stony plains which made a split 

 hoof a great handicap. There has been at the same time the 

 elaboration of a digestive system suited to hard dry food, and 

 the canines have atrophied because they were a handicap in 

 preventing the lateral movement of the lower jaw in grinding 

 the hard food. During the millions of years — Eocene to Plio- 

 cene — occupied in bringing about these changes, the direction 

 of the progress of the evolution of the horse has never changed, 

 and the weight of the animal has increased fifty-fold since he 

 had five toes. 



The chimpanzee falls in line with the horse in the growth 

 of the body, of the canine teeth, and probably in the size of 

 the ears, as compared with the apes of the Oligo-miocene. These 

 characters are all of manifest survival-value, and they have 

 all proceeded in the one direction. Both the horse and ape 

 illustrate Dollo's law. 



On the theory of the recent separation of man and ape 

 the anomaly of the ribs and backbone is another mystery. Man 

 has seventeen dorso-lumbar vertebrae and twelve ribs. The 

 gibbon, gorilla and chimpanzee have seventeen vertebrae and 

 thirteen ribs, the orang sixteen vertebrae and twelve ribs. 

 It is probable that the thirteenth rib is a character of direct 

 survival-value. Primitive races of mankind are like the apes 

 in possessing no waist, but they have only twelve ribs. The 

 thirteenth is occasionally found in man, but only as a rare 

 curiosity. Presumably then the recent common ancestor had 

 thirteen ribs and seventeen dorso-lumbar vertebrae. The rib 



