THE WONDER OF LIFE 529 



half of the eye is adapted for vision in air ; the lower 

 haK for vision under water. Now, however various the 

 individual variations in fishes' eyes, there is no evidence 

 that variations, which could by selection give rise to this 

 curious condition, occur in other species of fish. It seems 

 to me that we have no reason to suppose that the required 

 variations ever occurred until the ancestors of Anableps 

 took to swimming with their eyes half out of water. A 

 similar argument apphes to many other cases of special 

 adaptation, and the logical conclusion is that the habits 

 and conditions determined the modification'. 



This is admirably put and the diflSculty is great ; but it 

 should be noted that there has been very httle investiga- 

 tion of the variations in the eyes of fishes, that we have 

 very httle warrant for supposing that such a remarkable 

 change in the lens could arise as the direct result of the 

 pecuKar habits and conditions, and third, that it is possible 

 that the fish took to its pecuhar mode of surface swimming 

 because its pecuhar eyes were suited to that habit. 



Similar Structures put to Diverse Uses. — Our idea of 

 adaptability may be enriched if we consider how the same 

 structure is utihzed for all sorts of difierent results. Let us 

 take a series of glands, for instance, which, though not 

 quite homologous, are in a general way similar — pouring 

 a secreted juice into the mouth cavity. In a leech the 

 secretion keeps the ingested blood from coagulating, so 

 that it remains more usable in the crop ;■ in some marine 

 Gasteropods it contains dilute sulphuric acid which seems 

 to be of use in dissolving the carbonate of Hme, say in a 

 starfish's armour ; in some cuttlefishes, such as Eledone 

 moschata, it has a rapid paralyzing effect on the nervous 

 OTtern of c^abs which fojm an important part of the diet ; 



m;m 



