370 Dynamic Theory. 



the growth of a tree, or the putting out of leaves in the spring. The 

 first leaves to grow are those most favored by light or warmth or moist- 

 ure, and some will be quite formed before others are fairly begun. 

 Some law, too, limits the growth of each leaf, causing it to continue the 

 subdividing of the cells at certain places on the edge of the leaf, thus 

 expanding those portions and forming the projections and inequalities, 

 serratures and dentations to be found around the edges of the leaves. 

 Thus we may suppose that while the sap or protoplasm is sufficiently 

 diluted with moisture so as to be easily portable through the veins and 

 veinlets, and there is a proper amount of warmth and moisture, the leaf 

 will be larger, the marginal cells continuing to subdivide for a longer 

 time and more rapidly while they are at it. But if there is drouth, the 

 cell walls become hardened into woody tissue and resist subdivision. 

 As soon as extension in one direction receives a check from an} r cause 

 the nutriment is diverted to another direction and so the symmetry of the 

 pattern maintains itself. 



Darwin has called attention to the correspondence or correlation which 

 prevails between certain parts of an organism ; one part appearing to in- 

 fluence the development of another. Very often the reason of these 

 correlations is obvious, but sometimes it is obscure. 



Darwin observes that hairless dogs have imperfect teeth, and long 

 haired and coarse haired animals are apt to have long or many horns. 

 Pigeons with feathered feet have skin between their outer toes ; pigeons 

 with short beaks have small feet, those with long beaks large feet. 

 Hair, teeth, feathers, horns, claws, nails and beaks are all modifications 

 and products of the epidermis, and are all through it derived from the 

 same embryonic outer plate or skin sensory layer. It is possible that 

 this common origin may have something to do with the parity of devel- 

 opment in these common products. Some circumstance in the environ- 

 ment, as a severe climate, might stimulate development of the epider- 

 mis, which reflected back to the skin sensory layer of the embryo (in 

 reproduction) would give that layer an extra stimulation, and this in 

 the next generation might appear in several or all the products of that 

 layer. The correlation of long beaks with large feet in pigeons, like 

 that of long neck with long fore legs in the giraffe, might be due to a 

 mechanical necessity of the animal, supplemented by natural selection. 

 The longer the beak the greater the variation of the center of gravity, 

 which would tend to throw greater labor upon the feet with increase in 

 their development. Parity of parts is a constant subject of selection, 

 since when one part is subject more or less to another, the failure of one 

 is at least the partial failure of two. Another case that Darwin cites is 

 that "cats with blue eyes are invariably deaf," a very curious circum- 

 stance. Blue eyes are abnormal in a cat, and indicate an unhealthy 



