344 PERCY SLADEN TRUST EXPEDITION 



limited*. It is certain however that they vary greatly in this respect, the animals being 

 striped, spotted, or suffused in very different ways and with very different colours, 

 while the differences between species are often as great between species of the same genus 

 as between those of different genera. In the cases at present known the colours are 

 usually gaudy and conspicuous when the prawns are removed from their proper environ- 

 ment, though, in some instances at least, their colouration harmonizes very strikingly 

 with that of their natural surroundings. Nothing is known as to the colour changes 

 which doubtless occur in the lifetime of the prawns, or as to the way in which there 

 arises a correspondence between the colouring of the individual and that of its sur- 

 roundings, or as to the value this may have to the prawn. 



• 



On Adaptation. 



It will be clear, from various statements in the preceding pages, that our knowledge 

 of the significance of peculiar features in the Pontoniinae is at present very limited. 

 Yet it may not be useless even now to consider the problem they present, if only because 

 the alteration which is in progress in our views as to the mode of origin of such features 

 may well result in an underrating of their importance in the economy of the organism. 

 It has, for instance, quite lately been said that " to see fitness everywhere is mere 

 eighteenth century optimism." Now, as a repudiation of the conception of an ail-but 

 personified " environment," which by " selection " forces organisms into a mould pre- 

 determined by itself, this statement is undoubtedly justified. But it may also convey 

 a belief that an appreciable proportion of the characteristic features of organisms is not 

 correlated with peculiarities in their modes of life, and in this sense it is of much more 

 doubtful validity. 



There was a time when the structural peculiarities of organisms were widely held 

 to be due to the direct action of their environment in bringing them into harmony 

 with itself, by means of the plasticity of the individual and the inheritance of the 

 characters so " acquired." Thus every feature was shaped to meet some demand of an 

 environment which could not be escaped, and each played its part in ensuring the 

 viability of the species. That conception was replaced by one in which the organism 

 was allowed more initiative, in that it presented numerous small but ready-made 

 modifications to its environment, by which some of them were then " selected " to become 

 the characters of an altered race. This view had in turn to be modified in the direction 

 of attributing a greater importance to the organism itself, since it was discovered that 

 only some of the variations presented by the organism were heritable, and that these 

 were fixed and incapable of dilution or summation. Finally, it has to be recognized that 

 what is selected is not this or that " character," but the organism as a whole, with its 

 powers of adapting itself to the world by various means. Thus the part held to be 

 played by the environment in the production of any single character has become less and 

 less, and therewith has waned the belief that every character plays some part in the 



* Dana, Nobili, Potts, Rathbun and others give information concerning the colours of various species. 



