v CHANGE FROM A REPTILE TO A BIRD 57 



it cannot be too strongly insisted upon that they arc 

 not occasional but unceasing, so that it is highly 

 probable that two, that would be useless unless they 

 appeared simultaneously, might occur together in the 

 same individual. It is only reasonable to suppose 

 that some of the reptiles from which our birds have 

 sprung were born not only with a fore limb that, 

 fringed with scale-like feathers, might act as a make- 

 shift for a wing, but also, by a happy coincidence, with 

 hind legs stronger than those of their contemporaries. 

 Quite apart from such coincidences, a very slight 

 power of flight, due to a modification of the fore limb 

 not sufficient to incapacitate it for walking, would be 

 highly advantageous to the birdlike reptile, or reptile- 

 like bird. When menaced by a snake as he sat upon 

 a tree, he would flutter to another tree, perhaps feebly, 

 descending much as he went, his wings acting as a 

 clumsy parachute. Still, he would save his life. Here, 

 then, is a stage in advance accomplished. Why 

 not after this a development of the hind limbs, making 

 an upright posture possible ? And when this was 

 attained, why not a further development of wings ? 

 And why should not an improvement in the internal 

 organs follow closely upon the visible changes ? 

 The fact is that in this case it does not seem necessary 

 to assume an absolute simultaneity of variations. 



But supposing it is held by any one that modifica- 

 tions arising singly could not have advanced the 

 reptile to a bird, and that for simultaneous variations 

 we must not trust to mere coincidence, we must appeal 

 to what is called correlated variation. Many instances 

 of this are given by Darwin, Cats, which are entirely 



