0. — GEOLOGY. 81 



— either by sinking of the sea-floor or by migration of the organism. 

 Creatures already capable of becoming acclimatised will be the majority 

 of survivors, and among them those which change most rapidly will 

 soon dominate. Place your successive fornis in order, and you will get 

 the appearance of momentum ; but the reality is inertia yielding with 

 more or less rapidity to an outer force. 



Sometimes a change is exhibited to a greater or less extent by evei-y 

 member oi some limited group of animals, and this change may seem 

 to be coiTelated with the conditions of life in only a few of the genera 

 or species, while in others it manifests no adaptive character and no 

 selective value. Thus the loss of the toes or even limbs in certain 

 lizards is ascribed by Dr. G. A. Boulenger to an internal tendency, 

 although, at any rate in the Skinks, which furnish examples of all stages 

 of loss, it certainly seems connected with a sand-loving and burrowing 

 life. Recently Dr. Boulenger (1920, Bidl. Soc. Zool. France, xlv. 

 68) has put fonvard the East African Testudo lovendgcl^ a ribless 

 tortoise with soft shell that squeezes into holes under rocks, and swells 

 again like an egg in a bottle, as the final stage of a regressive series. 

 The early stages of this regression, such as a decrease in size of the 

 vertebral processes and rib-heads, were long since noticed by him in 

 other members of the same family; but. since they did not occur in 

 other families, nnd since he could perceive no adapiive value in them, 

 hi' regarded them as inexplicable, until this latest discovei-y proved them 

 to be pi'ophetic of a predestined goal. The slightness of my acquaint- 

 ance with tortoises forbids me to controvert this supreme example of 

 teleology as it appears to so distinguished an authority. But in all 

 these apparent instances we should do well to realise that we are still 

 incompletely informed about the daily life of these creatures and of 

 their ancestors in all stages of growth, and we may remember that 

 structiires once adaptive often persist after the need has passed or 

 has been replaced by one acting in a different direction. 



The Shidy of Adapiive Form. 



This leads us on to consider a. fruitful field of research, which would 

 at first seem the natural preserve of neontologists, bub which, as it 

 happens, has of late been cultivated mainly to supply the needs of 

 palaeontology. That field is the influence of the mode of life on the 

 shape of the creature, or briefly, of function on form; and, conversely, 

 the indications that form can give as to habits and habitat. For many 

 a long year the relatively simple mechanics of the vertebrate skeleton 

 have been studied by palaeontologists and anatomists generally, and 

 Have been brought into discussions on the effect of use. The investiga- 

 tion of the mechanical conditions controlling the growth of organisms has 

 recently been raised to a higher plane by Professor D'Arcy Thompson's 



parts through catalysera and hormones, then the process will involve lag varying 

 "with the physico-chemical constitution of the organism. Slight differences in 

 this respect between different races may have some bearing on the rate of 

 change {vide infra ' The Tempo of 'Rvnlntion '}. on the correlation of characters, 

 and 80 on the diversity of form. 



1920 a 



