BORRADAILE— ON THE PONTONIIN^ 345 



struggle for existence, since that is too often still regarded as the activity of a machine- 

 like organism in the face of a fixed and unavoidable environment. 



Now it is true enough that the structural " fitness " of an organism does not consist 

 in its characteristics having arisen in response to the fixed demands of an environment. 

 But this does not mean that each of them is not caused to play its part in that reaction 

 with the world which constitutes the life of the organism, including the habit of seeking 

 an environment and maintaining itself there by the necessary measures. These habits 

 are without doubt as modifiable, both in the individual and in the race, as any other 

 characteristic of the organism, and one of the conditions of the persistence of any given 

 structural modification is that it should not exceed the limits of the power of adjustment 

 which is given by this susceptibility of modification of habit. So long as the structural 

 alteration does not outrun the physiological possibilities, fitness is maintained, and it is 

 incredible that this can ever be due to the alteration being indifferent to the life of the 

 organism. Some disturbance it must cause, whether in the internal or in the external 

 physiology, and in the long run a disturbance in internal physiology will cause a modifica- 

 tion in behaviour, as in the amount of food taken, or in the sheltering of an animal which 

 by any cause becomes deprived of its agility. Any belief to the contrary can only arise 

 from the fact that the importance to the economy of the organism of any given structural 

 modification by no means necessarily corresponds with its conspicuousness in the eyes 

 of the human observer. 



It is, of course, also the case, though this is beside the present point, that alterations 

 in behaviour can only take place within limits set by the structure of the organism, 

 though it is not the case that every such change must involve a modification in the 

 structure of the organs used. Finally, it is still true that the organism must come to 

 terms with its world ; but that is not to say that it is dependent upon an inevitable 

 environment, or that it can maintain itself in a given set of surroundings only by a fixed 

 behaviour. Adaptation, in short, is a relation between structure and habit, and only 

 secondarily between structure and environment ; and there is no feature of an organism 

 the investigation of which can safely be regarded by the naturalist as without signi- 

 ficance, and relegated by him to the student of variation. 



The Distribution of the Pontoniinae. 



The majority of the Pontoniinee are members of the shallow water fauna of the 

 tropical and subtropical Indopacific. Six species occur in the Mediterranean, six on the 

 west coast of America, eight in the Western Atlantic, and one in South Australia. 

 Three of the Mediterranean species are also recorded from British waters. Urocaris 

 longicaudata and PalcBmonella tenuipes are stated to occur both in the West Indies and 

 in the Indopacific. Pontoniinse appear to be entirely absent from the colder seas. 

 Most are undoubtedly benthic, but very possibly a few of the more lightly-built forms, 

 such as Urocaris, will prove to belong to the plankton. One species of Urocaris is 

 bathypelagic, Coutierea lives in moderately deep water, probably on the bottom, Pal(B- 

 monella laccadivensis is dredged in 100 — 600 fathoms, and Periclimenes tenellus in 

 SECOND SERIES— ZOOLOGY, VOL. XVII. 44 



