between a Machine and its Model. 149 



sufficiency or to the economy of a structure. To enlarge or di- 

 minish the parts of a machine all in the same proportion, is to 

 commit a deliberate blunder. Let us compare the wing of an 

 insect with that of a bird : enlarge a midge till its whole weight 

 be equal to that of the sea-eagle, and, great as that enlargement 

 must be, its wing will scarcely have attained the thickness of 

 writing-paper; — the falcon would feel rather awkward with 

 wings of such tenuity. The wings of a bird, even when idle, 

 form a conspicuous part of the whole animal ; but there are in- 

 sects which unfold, from beneath two scarcely perceived covers, 

 wings many times more extensive than the whole surface of their 

 bodies. 



The larger animals are never supported laterally ; their limbs 

 are always in a position nearly vertical : as we descend in the 

 scale of size the lateral support becomes more frequent, till we 

 find whole tribes of insects resting on limbs laid almost horizon- 

 tally. The slightest consideration will convince any one that la- 

 teral or horizontal limbs would be quite inadequate to support 

 the weight of the larger animals. Conceive a spider to increase 

 till his body weighed as much as that of a man, and then fancy 

 one of us exhibiting feats of dexterity with such locomotive in- 

 struments as the spider would then possess ! 



The objects which I have hitherto compared have been re- 

 mote, that the comparisons might be the more striking ; but 

 the same principles may be exhibited by the contrast of species 

 the most nearly allied, or of individuals even of the same species. 

 The larger species of spiders, for instance, rarely have their legs 

 so much extended as the smaller ones ; or, to take an example 

 from the larger animals, the form of the Shetland pony is very 

 different from that of the London dray-horse. 



How interesting it is to compare the different animals, and to 

 trace the gradual change of form which accompanies each in- 

 crease of size ! In the smaller animals, the strength is, as it 

 were, redundant, and there is room for the display of the most 

 elaborate ornament. How complex or how beautiful are the 

 myriads of insects which float in the air, or which cluster on the 

 foliage ! Gradually the larger of these become more simple jn 

 their structure, their ornaments less profuse. The structure of 

 the bird? is simpler and more uniform, that of the quadrupeds 



