262 



Supplement to ''Nature'' August 18, 1923 



has become a fixed habit. This is but one instance 

 of what is often to be observed in the study of human 

 evolution, where an occasional variation in apes has 

 become the established form in man. 



How has this tendency to vary in a definite direction 

 arisen ? It is a direction wliich increases the functional 

 efficiency of the human leg. Has this tendency arisen 

 in apes as a result of the manner in which they chmb ? 

 Or is it, as Huxley would have us infer, a tendency 

 which is inherent in the developing soleus and has come 

 into existence under the influence of some unknown 

 factor which regulates the developmental movement 

 of muscle cells ? I think that Huxley's interpretation 

 is the true one. 



Let us take another example. Under the skin of 

 the sole of man's foot lies a muscle known as the 

 short flexor of the toes. In man all its four bellies, 

 designed for the outer .four toes, have a solid basis of 

 origin on the bone of the heel ; from such an origin 

 the collective muscle can play a helpful part in main- 

 taining the arch of man's foot. Now in the monkey's 

 foot, while the muscle for the second toe arises from 

 the lieel, the muscles for the three outer toes retain 

 a primitive origin from an adjacent surface provided 

 by moving tendons. In the gibbon it is usual for the 

 muscle of the third as well as of the second toe to 

 have moved its origin to the heel ; in the great anthro- 

 poids, particularly in the orang, the muscle of the 

 fourth toe has also left the tendon and migrated to 

 the heel ; in man all four have moved. Here we see 

 a human character arising as the culmination of a 

 tendency which can be observed, to a greater or less 

 degree, in the feet of all those animals which are most 

 closely related to man, yet more primitive than him 

 in structure. The migration of origin, on the part 

 of the embryonic muscular cells, is of a useful or 

 purposive kind. We cannot avoid the conclusion that 

 the growth and development of young muscle cells 

 are controlled by influences or means which work 

 towards a functional result. 



The Origin of a Muscle peculiar to Man. 



Man possesses a muscle which is peculiar to himself 

 — the peroneus tertius — and it will help us to under- 

 stand how new structural features have been, or are 

 being, evolved if we note the manner in which this 

 muscle makes its appearance during the development 

 of the leg and foot of the human embryo. The 

 peroneus tertius raises the outer border of the foot 

 and assists in applying the sole of the foot to the 

 inequalities of the ground in walking. If we examine 

 a hundred human legs we shall find ninety in which 

 the peroneus tertius is a complete and separate muscle, 

 but in the remaining ten we shall find some in which 

 it is separated only to a greater or less degree from an 

 adjacent and older muscle, the long extensors of the 

 toes, and some in which it is quite unseparated from 

 this muscle, as is the case in the legs and feet of 

 anthropoid apes. In the gorilla one notices occasionally 

 a tendency for the outer fibres of the tendon going to 

 the fifth or small toe to stray or migrate towards the 

 outer border of the foot. When we turn to the develop- 

 ing leg to ascertain how this new muscle makes its 

 appearance in the human embryo, we find, towards 



the end of the second month of development, that the 

 mass or colony of mu-scle cells whit h are to extend thi 

 toes, becomes separated from the common extensor 

 mass of the leg, and that, in turn, the mu.scle cells 

 which are to form the peroneus tertius separate or an 

 cleft from the outer side of the long extensors of th< 

 toes — from the part concerned in extending the liitK 

 toe and incidentally in turning upwards the outer 

 border of the foot. The j)eroneus tertius represents 

 a c(jlony of muscle cells which have broken away from 

 the parent muscle — the long extensor of the fifth toe. 

 The tendon fibres have broken away from those going 

 to the toe and migrated backwards along the outer 

 border of the foot, thus giving them an advantageous 

 position for the performance of their function in 

 walking. 



We have here all the properties manifested by develop- 

 ing muscle masses — a power of cleavage or separation, 

 and a power of migration. What causes these outer 

 muscle cells which are destined to act on the nn -1 

 external of the digits to break from the parent iiki- 

 and assume a separate functional identity ? I agret 

 with Huxley that there are no grounds for believing 

 that the behaviour of embryonic muscle cells is in 

 any way influenced by experiences gained by adult 

 muscle fibres. When vertebrate limbs came first into 

 existence the muscle colonies which deployed to form 

 the extensors of the toes, grouped themselves so as to 

 get a functional result. In the case of the outer toe 

 there was a double function, the extension of the toe 

 and the everting of the foot. In the human foot the 

 muscle cells which evert the foot have separated them- 

 selves from those which extend the little toe. The 

 evolutionary machinery hes in the bchaxiour of the 

 embryonic muscle cells or myoblasts. 



Instances of Parallel Inheritance. 



Let me cite two other examples which go to show 

 that myoblasts possess evolutionary tendencies which 

 work towards a purposive or functional end. The 

 interosseus muscles of the hand and foot of monkeys 

 arise, not from the adjacent surfaces of metacarpal 

 and metatarsal bones, as they do in man, but from the 

 bases of these bones, in the palm of the hand and sole 

 of the foot. In the human embr)'o the interosseus 

 muscles appear in the same palmar position as that 

 which is retained in monkeys. In the most primitive 

 of anthropoid apes — the gibbon, and also in the highest 

 of South American monkeys — the howler monkeys — 

 Ateles — the origin of the interosseus muscles have 

 migrated so as to take a partial hold of the adjacent 

 surfaces of the metacarpal and metatarsal bones. In 

 the great anthropoid apes — the gorilla, chimpanzee, 

 orang — and in man, these muscles ha\'e sunk in between 

 and seized the adjacent surfaces of the metacarpal 

 bones of the hand and metatarsal bones of the foot. 

 This migrator)' tendency has seized upon, or become 

 manifested in, the muscles of the hand as well as in 

 those of the foot, although these members are subject 

 to different functional influences. We can account 

 for such evolutionary' manifestations only by supposing 

 that in a remote common ancestor of all the members 

 of the higher primates there was a latent tendency 

 in the myoblasts of the interosseus muscles to deploy 



