THE EVOLUTION OE BIRDS. 



Whatever may be the cause of coincident variations, our ignorance of them cannot 

 blind our eyes to the effects. They exist, and must have existed from time immemorial. 

 The unknown causes may be few or many, and may even have changed, but the effects are 

 there, and must always have been there, otherwise the whole theory of Evolution falls to 

 the ground. 



The fact that variations do simultaneously appear in a large number of individuals in 

 the same locality, and are repeated for many successive generations, is the only foundation 

 upon which a theory of Evolution can be built, and, when once admitted, is all-sufficient. 



The basis of the theory of Evolution is the principle of descent with modification. To 

 take existing species of birds as an example, it assumes that every bird is a blood-relation 

 of all other birds. The Blackbird is a near relation of the Thrush, and it would not be 

 necessary to trace their genealogy very far back to arrive at their common ancestor. To 

 find the common ancestor of the Blackbird and the Swallow, it would be necessary to search 

 the records much further back ; and to find that of the Blackbird and the Snipe, it would be 

 necessary to go further back still ; whilst to find that of the Blackbird and the Ostrich, it 

 would be necessary to go back almost to the original species of bird from which all existing 

 species are descended. To produce these multitudinous modifications in the descendants of 

 ancient species, it is obvious that the apparent law that like produces like, that children 

 resemble their parents, must be modified in some way. The fact that children differ slightly 

 from their parents is as self-evident as that they bear a strong resemblance to them. The 

 variation of children from their parents is not accidental. Science does not admit the 

 existence of accident." So far as we know, children would exactly resemble their parents, if 

 they were subjected to precisely the same influence during the process of growth to puberty, 

 but this is impossible. The external influences of their surroundings are never precisely the 

 same ; climates change ; and the relation between a species and its supply of food varies 

 alternately with good or bad seasons, and is permanently affected by the natural increase of 

 popidation. The internal influences must also differ ; the transmitted experience, inherited 

 habits, or inherited memory, explain it as you will, call it what you like, are cumulative, so 

 that in this respect also every child stands on a slightly different platform from its parents. 

 The consequence is that no individual is precisely in the same position, when it produces 

 children, as its parents were, when it was produced. If the condition be a higher one, it is 

 obvious that if the process goes on long enough the accumulated slight variations of a great 

 number of generations must tend to an important advance in the species. The machinery 

 of Evolution is there ; the object of inquiry must be to discover the laws which govern the 

 direction of variation, which are consequently the laws which govern the origin of species. 



One cause of variation in an organ is the amount of use or disuse to which it is 

 subjected during the period of growth, whilst it is in a more or less plastic state. Practice 

 makes perfect. This law applies not only to accomplishments, such as riding, skating, 

 tight-rope dancing, or violin playing, but also to the organs required in the accomplishment, 

 provided that the practice begins early enough, before the organs have arrived at maturity. 



c 



Coincident 

 variations 

 necessary to 

 Evolution. 



And all- 

 sufficient. 



Descent 

 from com- 

 mon ances- 

 tors. 



Modifica- 

 tion often 

 the result of 

 external 

 influences. 



Sometimes 

 of internal 

 influences. 



Causes of 

 variation. 



