346 THE LAW OF EVOLUTION CONTINUED. 



size of them, and subsequent union of thein into an outer 

 layer, constitute the first differentiation; and the middle 

 area of this layer is rendered unlike the rest by still more 

 active processes of like kind. By such modifications upon 

 modifications, too multitudinous to enumerate here, arise 

 the classes and sub-classes of tissues which, variously in- 

 volved one with another, compose organs. 



Equally conforming to the law are the changes of gen- 

 eral shape and of the shapes of organs. All germs are at 

 first spheres and all organs are at first buds or mere rounded 

 lumps. From this primordial uniformity and simplicity, 

 there takes place divergence, both of the wholes and the 

 leading parts, towards multiformity of contour and towards 

 complexity of contour. Cut away the compactly- 



folded young leaves that terminate every shoot, and the 

 nucleus is found to be a central knob bearing lateral knobs, 

 one of which may grow into either a leaf, a sepal, a petal, 

 a stamen, a carpel : all these eventually-unlike parts being at 

 first alike. The shoots themselves also depart from their 

 primitive unity of form; and while each branch becomes 

 more or less different from the rest, the whole exposed 

 part of the plant becomes different from the imbedded 

 part. So, too, is it with the organs of animals. 



One of the Articulata, for instance, has limbs that are 

 originally indistinguishable from one another — compose a 

 homogeneous series; but by continuous divergences there 

 arise among them unlikenesses of size and form, such as we 

 see in the crab and the lobster. Vertebrate creatures equally 

 exemplify this truth. The wings and legs of a bird are of 

 similar shapes when they bud-out from the sides of the 

 embryo. 



Thus in every plant and animal, conspicuous second- 

 ary re-distributions accompany the primary re-distribution. 

 A first difference between two parts; in each of these parts 

 other differences that presently become as marked as the 

 first; and a like multiplication of differences in geometri- 



