78 GLIMPSES OF NATURE. 



always exhibited the slight differences on which our 

 ideas of their distinct and separate nature are founded. 



We are, all of us, anxious to pierce by the eye of 

 scientific faith the obscurity in which the past of living 

 beings lies hid. The thought is borne in upon us 

 day by day that, so far from species of animals being 

 " steady and stolidy " quantities, they are rather to be 

 regarded as being of very variable nature indeed. 

 For we know that living beings do vary and alter 

 some to a great extent, others scarcely at all, but one 

 and all showing a tendency or bias towards change. 

 If this idea (which I need hardly add is the basis of 

 all modern biological thought) be admitted as worthy 

 of further pursuit, it is not difficult to find many 

 examples of relationships among animals, and plants 

 also, which, in schoolboy language, appear as of a 

 decidedly " mixed " character. 



What, for example, are we to make out of the follow- 

 ing case, the facts of which are perfectly well ascer- 

 tained ? On oak-trees grow the galls which are used 

 in ink-making and in medicine. A gall is an excres- 

 cence, which, as most of my readers know, is due to 

 the work of an insect. The gall-flies (of which the 

 best-known group is that called Cynips) thus derive 

 their popular name from their habit of gall-production. 

 The mother gall-fly possesses a hollow pointed tube 

 which is known as the ovipositor. Down this tube 

 pass the eggs which she fixes on the oak-tree. The 

 gimlet-like ovipositor pierces the tree's bark, and some 

 irritating fluid is doubtless injected into the plant- 

 tissues along with the egg. 



At any rate, the tree swells at the seat of the puncture, 

 and soon the gall appears ; the excrescence developing 

 in this way as the investment of the young insect 



