994 LIFE : OUTLINES OF GENERAL BIOLOGY 



often seen in the fields are mainly to the good, since they are 

 predominantly insectivorous, and destroy huge numbers of injurious 

 pests. 



Slow Changes of Intelligent Behaviour. — It is easy to 

 put a date-limit to the Kea parrot's change, namely the introduction 

 of sheep into New Zealand. There are also many observations to 

 show that the Herring Gull and the Lesser Blackback have changed 

 their diet in increasing numbers of recent years. But there are other 

 cases of similar changes which must have taken a much longer time. 

 The Robber Crab (Birgus latro), common on Christmas Island and 

 the like, is a giant relative of the hermit-crabs in shore waters. But 

 it has become terrestrial, and does not require to go to the sea 

 except to spawn. This change of habitat from water to land must 

 have required a very long time, for it is associated with a change 

 in the breathing organs — a change which could not be effected 

 quickly. The Robber Crab has to breathe atmospheric air, not air 

 dissolved in water, as ordinary Hermits do. But what we are con- 

 cerned with at present is something different — namely, the Robber 

 Crab's novel habit of climbing the stems of the Coco-palms in order 

 to secure the nuts. With its heavy claws the crustacean makes a hole 

 in the nut and spoons up the flesh. The broken shell may be after- 

 wards used, in Hermit Crab fashion, as a shelter for the tail. Now 

 this extraordinary habit of climbing the Coco-palm could not, we 

 think, be learned in a day; and yet it must have been learned, and 

 not so very long ago, for the Coco-palm is not a native of these 

 parts. The Coco-palms of Christmas Island must have sprung from 

 nuts wafted by currents from distant shores. Let us be clear, at this 

 point, that Hermit Crabs have an instinct — an inborn, hereditary, 

 racial predisposition or prompting — to shelter their vulnerable 

 sensitive tail in some borrowed house, usually the shell of a whelk 

 or some other sea-snail. The Robber Crab's tail is not soft as in 

 Hermits, yet Birgus is true to the ancestral instinct to seek some 

 extrinsic shelter, such as the personally secured shell of a coco-nut, 

 or the fortuitously discovered empty beef-tin. Such is an instinctive 

 prompting, but the climbing of the Coco-palm is surely an intelligent 

 response to a new situation. We take it as an instance of a new 

 habit adopted intelligently, but probably requiring generations of 

 testing before it became part of the orthodox routine of life. Organic 

 Evolution is a long drawn-out commentary on the text: Test all 

 things and hold fast to that which is good. 



The case of the Robber Crab and the Coco-palm seems to us to 

 illustrate well what is so often ignored in conventional aetiology, 

 that all robust, resolute animals share in their own evolution. They 

 are not inert items being sifted against an inexorable sieve, they are 

 also individualities which struggle and strive. They are not mere 

 pawns, moved by a callous Environment; they are living creatures 



