236 HABIT AND INTELLIGENCE. [cHAP. 



tte forms IS the Same, altliougli there may be conspicuous external 

 g jgg^'"® unlikeness, as there is between the maggot and the fly, 

 between the winged male and the wingless female of the 

 glow-worm, and between a flov/er and a leaf-bearing axis.-^ 

 "We might Tliis is what might be expected. We need not seek for 

 thL any special adaptation to purpose in the fundamental 



resemblance between forms that belong to the same spe- 

 cies ; ' it would rather be necessary to seek for some special 

 purpose, if we found a fundamental difference. 



There is, however, a well-known and remarkable in- 

 stance, and there are probably many others of the kind, 

 where the resemblance in external characters between the 

 sexes is carried further than is in any way needed by the 

 Nipples in purposes of life. I speak of the existence of nipples, which 

 ^^'^^' are, in fact, rudimentary organs of lactation, in the male 



of the human species. These have no function, and con- 

 sequently their existence cannot be referred to the law of 

 the adaptation of structure to function : it must be due to 

 a quite independent formative law.^ 

 Eelations The same is true of many of those relations of which I 

 the indi- have now to speak, between homologous parts of the same 

 yuliial. individual, and even between parts which are not homo- 



Homology ^ . 



of hands logous. Thus, the hands and the feet in man are evidently 



and feet, homologous, not Only in their relation to the rest of the 



skeleton, but in the number and position of the bones. 



The resemblance is carried much further than the law of 



adaptation to purpose will account for. That law will not 



account for the fact that there are both five fingers and 



five toes. If the hand needs five fingers for its uses, and 



no more, the foot might surely have done perfectly well 



Their with fewer than five toes. As I have mentioned in the 



tendency chapter on Variation, the hands and the feet tend to vary 



together, together : thus, persons with large hands usually have 



1 I say a leaf-bearing axis, not a leaf, because the flower consists of a 

 number of modified leaves, and conser[uently is homologous with an axis 

 bearing a number of leaves, not with a single leaf. 



^ A perfectly parallel case is that of the Hibernia leucophaearia, an 

 insect of which the male is winged, whUe the female is "nearly, though not 

 entirely apterous, the rudimentaiy wings being distinctly visible with a 

 good magnifier." (Mr. Inchbald, in the Zoologist, 1848, p. 2151.) 



