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Thus a difficulty presents itself as to the explanation of the three following relationships: That of the Pterodactyles with carinate birds; that of the Dinosauria with struthious birds; that of the carinate and struthious birds with each other.
Ink in the notes on the least prominent parts of the flint, in small capital letters, when in camp, with waterproof China ink. Styles of flint work. The Eoliths are worn pebbles, chipped as if for scraping. The Rostro- carinate flints found at the base of the Crag are long bars with a beak-end, suited for breaking up earth. The human origin of both of these classes is contested.
But to this it may be replied that the ancient common stock could not have had at one and the same time a shoulder structure of both kinds. It must have been that of the struthious birds or that of the carinate birds, or something different from both. If it was that of the struthious birds, how did the pterodactyles and carinate birds independently arrive at the very same divergent structure?
If it was that of the carinate birds, how did the struthious birds and Dinosauria independently agree to differ?
That merely minute, indefinite variations in all directions should unaided have independently built up the shoulder structure of the pterodactyles and carinate birds, and have laterally depressed their optic lobes, at a time so far back as the deposition of the Oolite strata, is a coincidence of the highest improbability; but that an innate power and evolutionary law, aided by the corrective action of "Natural Selection," should have furnished like needs with like aids, is not at all improbable.
It has been suggested to me that this difficulty may be evaded by considering pterodactyles and carinate birds as independent branches from one side of an ancient common trunk, while similarly the Dinosauria and struthious birds are taken to be independent branches from the other side of the same common trunk; the two kinds of birds resembling each other so much on account of their later development from that trunk as compared with the development of the reptilian forms.
Finally, if it was something different from either, how did the carinate birds and pterodactyles take on independently one special common structure when disagreeing in so many; while the struthious birds, agreeing in many points with the Dinosauria, agree yet more with the carinate birds? Indeed by no arrangement of branches from a stem can the difficulty be evaded.
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