3 oo THE VARIATION OF ANIMALS IN NATURE 



solve it by assuming that effects of the organs on one another 

 during use are inherited. 



(3) The origin of habits. — It has not rarely been assumed 

 that if we can show that some insignificant structure is defi- 

 nitely related to some part of the normal habits of the animal, 

 that structure has been proved to be adaptive. A little analysis 

 of a few concrete cases, however, reveals that this is a rather 

 naive assumption unless the adaptive nature of the habit 

 itself is proved. Before conducting these analyses a short 

 consideration of the relation of structure to habits is desirable. 



Woodger (1929, chapter vii) has endeavoured to show 

 that the antithesis often drawn by biologists between function 

 and structure is false — that the two are only aspects of one 

 entity, structure alone being a mere abstraction of the anatomist, 

 who ignores the element of time which is really inseparable 

 from a living organism. Woodger's argument appears incon- 

 testable when applied to any of the intricate internal adapta- 

 tions which are characteristic of living organisms. Even in a 

 simple case, as when a structural change in the eye of an 

 insect alters its phototropic response, it is illogical to speak of 

 structure determining function or vice versa. But the case is 

 different with many of the small structural or habit differences 

 which distinguish species. Thus in the Psammocharidae (dealt 

 with on p. 276), species either with or without a 'tarsal comb' 

 may burrow in sand, and it is quite reasonable to inquire 

 whether (a) the development of a comb enabled certain species 

 to restrict themselves to looser soils, or whether (b) only 

 certain of the species which had adopted loose soils for their 

 habitat were able to develop a comb for digging. On our 

 present knowledge we cannot actually decide between these 

 alternatives. 



A rather different example is given by Edwards (1929, 

 pp. 35-6) in his account of the flies of the family Blepharo- 

 ceridae. Here, in several genera, the mandibles are present 

 in the females of some species, absent in others. The species 

 with mandibles are blood-suckers, those without them visit 

 flowers. The mandibles are always found fully developed or 

 absent, never in an intermediate condition. It appears in 

 this case that the presence or absence of the mandibles (struc- 

 ture) determines habit, for species without mandibles could 

 never adopt the blood-sucking habit. 



