50 THE PRESIDENT'S ADDRESS. 



removed. This would be a perfectly good reason if I were 

 addressing a more deeply-learned Society, but ours is one where 

 we need not be ashamed of amusing ourselves by talking over 

 some of the interesting developments of life, even if they be gene- 

 rally admitted. 



It seems to me that the one great fact which proves evolution, 

 and which cannot reasonably be explained in any other way, is the 

 almost universal tendency of different forms of life to run into 

 series, both as to species and genera and as to the adaptation of 

 homologous organs. It is this only which enables us to classify 

 fauna and flora as we do. Take almost any order of plants or 

 animals, it matters not what, and we find a great number of more 

 or less similar forms, similar not merely in the great facts of their 

 structure, but in all sorts of even trifling points ; these points being 

 more developed in one species than in another, sometimes almost 

 rudimentary, but almost always present in some degree. Take 

 up the table for identification of species in almost any monograph, 

 and it will strike you at once how the ingenuity of the author has 

 been taxed to find tangible distinctions between closely-allied 

 species, and how he is enabled to work down from the general to 

 the particular by most of the species usually having many, even 

 trifling, characters in common. The Diptera are excellent ex- 

 amples ; take up Walker's Diptera and see what occurs even 

 with the British species : try to identify a gnat which you do not 

 know ; you will find a long series of species so similar that the 

 table for identification is appalling. Again, why should the 

 possession of a bi-lobed tongue furnished with pseudo-trachea be so 

 much correlated with two effective wings and two poisers, there 

 are plenty of other forms of sucking mouth ? Why should almost 

 all Sphinx larvae have a curved spine in the middle of the last 

 segment of the body ? Why should the number of spines on 

 the respective segments of the larva of all the species of 

 Vanessa butterflies be almost constant ? Why should the trachese 

 be developed into leaf-like organs, the so-called lungs, in all 

 scorpions and so many spiders and not in anything else ? Why 

 should two parallel blade-like ridges (the lamella?) run along the 

 cephalo-thorax of about half the Oribatidce, each ridge bearing a 

 hair at its end ? Why should a trace of the ridge be found in 

 almost all the other species, and why should the hair sometimes 

 persist in the instances where the ridge cannot be detected ? 



