S48 THE DESCENT OF MAN 



traced by its track of slime over a wall into an adjoining 

 well-stocked garden. Mr. Lonsdale concluded that it had 

 deserted its sickly mate; but after an absence of twenty-four 

 hours it returned, and apparently communicated the result 

 of its successful exploration, for both then started along 

 the same track and disappeared over the wall. 



Even in the highest class of the Mollusca, the Cepha- 

 lopoda, or cuttlefishes, in which the sexes are separate, 

 secondary sexual characters of the present kind do not, as 

 far as I can discover, occur. This is a surprising circum- 

 stance, as these animals possess highly developed sense- 

 organs and have considerable mental powers, as will be 

 admitted by every one who has watched their artful endeav- 

 ors to escape from an enemy.' Certain Cephalopoda, how- 

 ever, are characterized by one extraordinary sexual character, 

 namely, that the male element collects within one of the 

 arms, or tentacles, which is then cast off, and, clinging 

 by its sucking- disks to the female, lives for a time an inde- 

 pendent life. So completely does the cast-oflf arm resemble 

 a separate animal, that it was described by Cuvier as a para- 

 sitic worm under the name of Hectocotyle. But this mar- 

 vellous structure may be classed as a primary rather than 

 as a secondary sexual character. 



Although with the Mollusca sexual selection does not 

 seem to have come into play, yet many univalve and bivalve 

 shells, such as volutes, cones, scallops, etc. , are beautifully 

 colored and shaped. The colors do not appear in most cases 

 to be of any use as a protection ; they are probably the direct 

 result, as in the lowest classes, of the nature of the tissues; 

 the patterns and the sculpture of the shell depending on its 

 manner of growth. The amount of light seems to be influ- 

 ential to a certain extent; for although, as repeatedly stated 

 by Mr. Grwyn Jeffreys, the shells of some species living 

 at a profoiind depth are brightly colored, yet we generally 

 see the lower surfaces, as well as the parts covered by the 



5 See, for instance, the account which I have given in my "Journal of 

 Eesearches, " -1845, p. 1. 



