ch. iv. THE LAW OF EVOLUTION. 347 



The same kind of integration may be traced in the nervous 

 systems of insects ; and the reproductive system of the verte- 

 brata furnishes like instances of coalescence which are so 

 conspicuous that they are now usually made one of the 

 primary bases of classification in this sub-kingdom. The 

 reason why Von Baer overlooked this essential process, is pro- 

 bably to be found in the fact that each secondary integration, 

 resulting in increased definiteness, serves to make the accom- 

 panying differentiation still more prominent. The differen- 

 tiation of lungs, for instance, from the outer coat of the 

 endoderm, becomes marked in proportion as the flower-like 

 buds become integrated into organs of definite contour. But 

 while the two correlative processes go on hand in hand, it 

 is none the less true that they are distinct processes, and 

 that a comprehensive formula of evolution must explicitly 

 describe them both. 



In further illustration of this twofold aspect of evolution, 

 we may cite a fact which will by and by be seen to have 

 other important bearings, but which may here serve as a 

 valuable appendix to the foregoing discussion. This is the 

 fact that, in ranking different organisms as high or low in 

 the scale of life, we always proceed chiefly with reference to 

 the degree of heterogeneity, definiteness, and coherence which 

 they exhibit. Those plants and animals which we rank as 

 lowest in the scale are simply cells, like the homogeneous 

 cells from which higher plants and animals are developed. 

 So little specialized are these forms that they do not exhibit 

 even those characteristics by which we ordinarily distinguish 



raents in conformity to the requirements of the aggregate are almost endless 

 in variety, the extremes, hoth of concentration and of specialization, being 

 seen in the ant, the spider, and the crab. In many of the details of this 

 gradual fusion of distinct individuals into a coherent whole, we see the hypo- 

 thesis interestingly illustrated and justified. In the annelids of low type, 

 each segment has its own spiracles which have no internal communication 

 with one another. On the other hand, in the insect-group there is a com- 

 plete system of vessels connecting the respiratory systems. While in the 

 intermediate niyriapoda we find, as might be expected, a partial commuui* 

 cation. 



