342 SCIENCE PROGRESS. 



behind which came, as in the insects, a thoracic region with 

 three pairs of hmbs. 



Here it will be seen that the whole stress is laid upon 

 the quantitative differentiation of the segmentation ; whereas 

 as soon as the qualitative differentiation of the anterior 

 segments of the two groups are compared it is seen at once 

 that, in spite of all remarkable resemblances, the Insecta 

 and the Arachnida have nothing in common except descent 

 from a Chaetopod Annelid. 



The initial specialisation of the ancestral insect was un- 

 doubtedly also an adaptation to the method of feeding 

 adopted. Unlike the Arachnids, the food was probably 

 from the first supplied by the vegetable kingdom, and the 

 limbs nearest to the mouth (except the first pair which be- 

 came feelers) were specialised into biting jaws for tearing 

 and crushing the edges of leaves or the surfaces of stalks. 

 Highly developed locomotory powers were hardly needed 

 for this method of feeding, so that an annelidan stage with 

 only a head region showing the typical specialisation of the 

 Insecta long persisted, and still persists, in the grub or 

 caterpillar stages. We have no caterpillar or grub stage 

 in the Arachnids because, being from the first carnivorous, 

 the necessity of catching their prey required the develop- 

 ment of highly specialised locomotory powers concurrently 

 with the adoption and perfection of their method of feeding. 



In this case again, then, the analytical method is em- 

 phatic against there having been any close affinity between 

 the Arachnida and the Insecta. The three chief groups of 

 the Arthropoda, the Insecta, the Crustacea and the Arachnida 

 must be regarded as separate and distinct derivatives of the 

 Chaetopod Annelids. 



I should like to add in conclusion that in thus insisting 

 upon the necessity of solving the questions of affinity be- 

 tween the Arthropods by appeal to their " essential mor- 

 phologies," by which I mean the peculiar structural modi- 

 fications of their segments treated as physiological adapta- 

 tions, I am aware that the method is not a new one ; it lay 

 in the direct path of morphological research. It needs 

 emphasising, however, because the hopes which the study 



