INTRODUCTION 23 



Darwin independently and at a somewhat late day discovered this 

 essential principle as told in his Autobiography:^ 



" But at that time I overlooked one problem of great importance; and it is 

 astonishing to me, except on the principle of Columbus and his egg, how I could have 

 overlooked it and its solution. This problem is the tendency in organic beings 

 descended from the same stock to diverge in character as they become modified. 

 That they have diverged greatly is obvious from the manner in which species of 

 all kinds can be classed under genera, genera under families, families under sub- 

 orders and so forth; and I can remember the very spot in the road, whilst in my 

 carriage, when to my joy the solution occurred to me; and this was long after I . 

 had come to Down. The solution, as I believe, is that the modified offspring of 

 all dominant and increasing forms tend to become adapted to many and highly 

 diversified places in the economy of nature." 



The writer has termed this principle of embranchement of Lamarck, 

 or of divergence of Darwin, the law of adaptive radiation.'^ According to 

 this law each isolated region, if large and sufficiently varied in its topog- 

 raphy, soil climate, and vegetation, will give rise to a diversified mam- 

 malian fauna. From primitive central types branches will spring off in 

 all directions with teeth and prehensile organs modified to take advan- 

 tage of every possible opportunity of securing food and in adaptation of 

 the body, limbs, and feet to habitats of every kind, as shown in the 

 diagrams on page 24. The larger the region and the more diverse the 

 conditions, the greater the variety of mammals which will result. 



The most primitive kinds of mammals were probably small insec- 

 tivorous or omnivorous forms, therefore with simple, short-crowned teeth, 

 of slow-moving, ambulatory, terrestrial, or arboreal habit, and with short 

 feet provided with claws. 



In seeking food and avoiding enemies in different habitats the limbs 

 and feet radiate in four diverse directions; they either become fossorial 

 or adapted to digging habits, natatorial or adapted to amphibious and 

 finally to aquatic habits, cursorial or adapted to swift-moving, terrestrial 

 progression, arboreal or adapted to tree life. Tree life leads as its final 

 stage into the parachute types of the flying squirrels and phalangers, or into 

 the true flying types of the bats. We have not thus far found a single 

 instance in which a mammal is known to have been transformed from an 

 aquatic into a land type; it is always the reverse. Nor have we found an 

 instance where the extreme fossorial or cursorial types have retrogressed 

 into slow-moving, ambulatory, or terrestrial types. There is some evi- 

 dence, however, of arl)oreal types secondarily taking up terrestrial habits, 

 as in the case of many of the terrestrial and cursorial marsupial mammals 

 of Australia, which are believed to have evolved from specialized arboreal 



' The Life and Letters of Charles Darwin, including an Autobiographical Chapter. 

 Edited by his son Francis Darwin. London, 1888, Vol. I, pp. 68-69. 



' Osborn, The Law of Adaptive Radiation. Ajner. Natural., Vol. XXXIV, 1902, pp. 353- 

 363. 



