270 THE VARIATION OF ANIMALS IN NATURE 



(b) There is a distinct tendency to argue from structure to 



habitat in explaining the origin of many modifica- 

 tions. 



(c) The paucity of actual numbers of specimens of each 



species obtained makes it very difficult to reason as 

 to the distribution of these animals and the relation 

 of their structure to their habitat. 



(d) A study of one particular group, viz. the Octopoda, 



impresses on one the apparently capricious incidence 

 of modification apparently related to the abyssal 

 habitat. 



(e) Certain of the deep-sea forms exhibit modifications 



involving the loss or reduction of given structures 

 (e.g. in the Cephalopoda, of eyes, the ink-sac, the 

 musculature). The difficulties involved in a selec- 

 tive explanation of the loss of a given structure 

 are considered on p. 42. Whether these anomalies 

 will be removed by a more intensive study is very 

 uncertain. It is enough for the present to cite the 

 capricious modification of the eyes among species of 

 the same genus (Robson, 1925 ; Brauer, 1908, 

 p. 256 ; Murray and Hjort, 191 2, pp. 680-5). 

 •It is quite safe to state that, between littoral or 

 shallow-water species and those of the same genus 

 found at greater depths, structural differences may 

 often be found (e.g. among the Octopoda the reduc- 

 tion of the musculature in Benthoctopas berryi com- 

 pared with that of B. piscatorum). Nevertheless, as 

 between the shallow-water and the abyssal forms 

 it is impossible to formulate any hard and fast 

 diagnosis, and amongst those inhabiting deep water 

 we find some displaying particular modifications 

 and others which do not (cf. Grimpoteuthis glacialis 

 (Robson, 1932a, p. 28)). 



(4) Cave animals.— As regards the characters of cavernicolous 

 animals the position is quite different from that discussed in 

 (3). Thanks to the labours of Racovitza, Jeannel and others, 

 the distribution, modification and conditions of life of these 

 forms have been thoroughly investigated. The study of the 

 origin of the special modifications, however, labours under a 



