2go THE VARIATION OF ANIMALS IN NATURE 



striatus, and in ' tropical forest with much undergrowth ' 

 C. guttatus guttatus, which differs from the former in the marked 

 increase of spotting. This difference is connected (I.e. p. 121) 

 with ' the different features in the distribution of light in the 

 various terrains on which these lizards live.' His primary 

 contention is that there is a direct influence of the amount of 

 light on the distribution of pigment in the skin ; but (p. 122) 

 he also contends that there may be a selective advantage in 

 having, e.g., a broken pattern in habitats where the light is 

 broken by the characteristic vegetation. He expresses a 

 doubt (p. 123) whether selection can act in this way ; but 

 he stresses the fact that differences between the juvenile 

 and adult livery seem to be related to differences in habitat 

 noticed between young and adult forms of the same species. 

 This is a highly interesting case, in so far as the author 

 attempts to find an environmental basis for what would other- 

 wise pass as an ' orthogenetic ' series. It is, however, impossible 

 to judge the value of his suggestion, as his data are not statis- 

 tically treated and the incidence of the various types in the 

 particular habitats is not expressed numerically. 



15. Ovipositors of Noctuid moths (Edelsten, 190J). 



Edelsten records that in the two Noctuid moths Nonagria 

 cannae and JV. sparganii the ovipositors differ, being adapted in 

 one species to pierce plant-tissues and in the other to roll up a 

 leaf, so that the egg can be laid on the under-side. There is no 

 indication as to why one form of oviposition is better than the 

 other. Doubtless the difference in ovipositor is necessary, but 

 can we say the same for the habits ? (Cf also p. 300.) 



16. Teeth o/"Varanus niloticus (Lonnberg, igoj). 

 Lonnberg states that most species of Varanus (lizards) have 



sharp, pointed teeth, but V. niloticus, which appears to be 

 exceptional in feeding on Mollusca, has blunt teeth adapted to 

 crushing them. Similar observations have been made on the 

 teeth of fishes ; but it is far from clear to what extent allied 

 species are distinguished by such differences. 



The great defect in most evidence of this kind is (a) the 

 casual and anecdotal nature of the evidence, (b) the failure to 

 show that the correlation between structural diversity and habit 

 is of wide occurrence within given groups, and (c) the general 



