EVOLUTION— PYCRAFT 225 



The spatulate tail suffices for the still water, but life in the sea 

 demands more strenuous movemente, hence the change from spatulate 

 to triangular. 



Finally we have the case of the penguins. And these must be 

 considered with the guillemots, and razor-bills, which, you will 

 remember, use their wings as propellers under water. But these 

 have not become "flippers", because they are used yet more inten- 

 sively for flight, up to their breeding ledges, and down to the sea. 

 The great auk lost the power of flight because it bred on low ground, 

 which could be reached without the aid of wings. The decline in 

 the size of the wings would probably have gone no further because 

 ihey were needed as propellers. The penguins are still more inten- 

 sively^ aquatic, and have no need for wings to carry them to nesting 

 ledges on steep cliffs. Hence they gi^adually assumed the form of 

 flippers, their size depending exactly on the measure of their use. 



And now let me pass to a few cases which seem, so to speak, to 

 "floodlight" the problem of "use and disuse", that is to say of effects 

 of persistent stimuli concentrated on one organ, or part of an organ. 

 These illustrate what I would call "reciprocity" in development, and 

 are shown on plate 3. 



There are some birds Avherein the windpipe, for some mj^sterious 

 reason, develops "hypertrophy," It grows too long for the neck. In 

 the African black guinea fowl {Guttera)^ as a consequence, it forms a 

 loop which came to be apposed to the hypocleidum — the flattened 

 plate formed at the junction of the two clavicles, or "wish-bone." As 

 a consequence of this contact, this plate first developed a thickened 

 edge at the point of contact, which gradually expanded to form a 

 cup, into which the loop is received. In some swans, and again in 

 some cranes, a still more exaggerated lengthening of the windpipe 

 brought about the formation of a loop which established contact 

 with the anterior border of the keel of the sternum. This, as you 

 know, forms a median plate running down the center of its under 

 surface. It is formed of two thin layers of bone enclosing cancel- 

 lated tissue between them. Now when the loop began to press, ever 

 so lightly, against the front border, it "splayed out" to form a 

 shallow trough. As the pressure steadily increased this trough deep- 

 ened, forcing apart the walls of this keel, till, at last, a long tunnel 

 was formed for the reception of this excess in the length of the 

 windpipe. It attains its maximum development in the whistling 

 swan, where the tunnel expands to invade the bony tissue of the 

 hinder end of the body of the sternum itself. Here we have two 

 entirely different organs reacting in a reciprocal manner. Surely 

 no better, or more convincing illustration can be found in support 

 of my contentions as to the effects of persistent stimuli in molding 



