202 LECTURES ON 



aspect of a brace turned the wrong way — thus ■ — ^—^ ; a very good 

 specific distinction, if no intermediate specimens had been found. A 

 series of deck margins, belonging to this and the following species, 

 will be found represented on plate 8 of the British Association report, 

 figs. 1/., Sg. 



Fig. 2. 



The best means of 

 distinguish i n g the 

 species of slipper- 

 limpets from each 

 other was found to be 



Deck margins of Crepiilula aruleata. The straight line represents the situa- l'U6 Snape 01 ILIB nu- 



tion of the medial nb. clear portion and the 



mode of growth of the very young shell. Whatever be the abnormal 

 character of the adult, it did not appear that the offspring had a ten- 

 dency to the same degeneracy, but rather to the resumption of the 

 normal type. In the case of local varieties, the peculiarities are repro- 

 duced, because they depend on circumstances which affect all alike. 

 But in such cases as those under consideration, where the extremes 

 and all the intermediate forms of variation are found in the same local- 

 ity, the changes depending on the accidents of the individual, it is not yet 

 proved that the idiosyncracies are transmitted. In fact, the frequent 

 instances in which the individual itself changes its form and sculp- 

 ture at different periods of its life is against such a hypothesis. In 

 the higher animals, where there is, as it were, an innate vital power 

 shaped according to the species, with an additional power shaped ac- 

 cording to the individual, and these powers are to no slight extent 

 irrespective of the immediate external surroundings, there is a much 

 stronger tendency in the offspring to imitate the parent — as in the 

 black faces of the Southdown sheep, or the stump -tailed cats of the 

 Isle of Man. This tendency on the part of the parent to reproduce 

 itself, and even that particular phase of self which obtains at the pe- 

 riods of conception and gestation, culminates in man; who, of all ani- 

 mals, is the most independent of external circumstances. But, in the 

 lower forms of life, the nature both of the species and the individual 

 becomes more and more plastic, responding to the accidents of the 

 moment; there is accordingly proportionally less of the innate power 

 which leads to the transmission of variety. As instances of this plas- 

 ticity the reader is referred to Dr. W. B. Carpenter's papers on Or- 

 bitolites and other forms of Rhizopods in the Transactions of the Royal 

 Society of London. 



It is a fact worth noticing, that while some species of shells are 

 extremely variable, others, inhabiting the same localities, are very 

 constant in their characters. These are seldom widely diffused, and 

 are often rare in individuals. A few young specimens of such spe- 

 cies were found among the slipper-limpets on the Spondyli; but the 

 bulk of the specimens belonged either to C. aculeata, which, as we 

 have seen, is a somewhat ubiquitous species, or to C. nivea, which, 

 under many shapes and many names, spreads over the principal part 



