THE LAW OF EVOLUTION 



furnishes like instances of coalescence which 

 are so conspicuous that they are now usually 

 made one of the primary bases of classification 



forming different functions in concert. Hence we obtain the 

 lowest actual type of annul oid, in which the segments are mere 

 repetitions of each other, with the exception of the extreme 

 front and rear segments, which subserve different functions 

 related to the welfare of the aggregate. 



Viewed in this light, the various great classes of the annulose 

 sub-kingdom beautifully illustrate that progressive coordination 

 of parts becoming more and more unlike one another, which 

 is the chief characteristic of Evolution as displayed in the or- 

 ganic world. In very low annelids, such as the intestinal 

 worms, we see hardly any specialization among the parts ; and 

 as we proceed upwards through the lower types, ending with 

 the myriapoda, we meet with a great but varying number of 

 segments, which show but little specialization save in the head 

 and tail. The same is true in general of the larvae and cater- 

 pillars of the higher types. But as we rise to the adult forms 

 of the insect-group — comprising crustaceans, arachnoids, and 

 true insects — we find the number of segments reduced to just 

 twenty. And while this number remains unvarying, the mod- 

 ifications undergone by different segments in conformity to the 

 requirements of the aggregate are almost endless in. variety, the 

 extremes, both 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 co- 

 herent whole, we see the hypothesis 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 complete system of vessels connecting the respiratory sys- 

 tems. While in the intermediate myriapoda we find, as might 

 be, expected, a partial communication. 



2 35 



