48 Aids to Microscopic Inquiry. 



course. The force exerted by water currents, which is con- 

 siderable at low velocities, determines the kind of organism that 

 can live in certain situations. In ponds which are only rippled 

 on the surface, and in streams that flow quickly, the conditions 

 are so different, that if similar creatures are found in both of 

 them, it is because, like the caddis worms who build their 

 houses heavy or light, they can accommodate themselves to 

 circumstances of different kinds. Still, or gently moving 

 waters afford the conditions most favourable to delicate and 

 minute forms of life ; biit the torrent should not be neglected, 

 as it often rushes over stones to which Confervse cling, and in 

 the shelter of such friendly vegetation even feeble organisms 

 may dwell in peace, and enjoy the large supply of air that 

 foaming water contains. 



The strength of a body depends on the cohesion of its par- 

 ticles, and still more on their arrangement and position. In 

 tall objects like towers, factory chimneys, or trees, the breadth 

 or thickness sufficient for one of fifty feet elevation will not 

 suffice for another of a hundred, and in a horizontal beam the 

 thickness that will give strength with a projectiou of ten feet 

 from the point of support, will not do for twenty. Nor is it 

 sufficient that the thickness should increase in simple propor- 

 tion to the height or length. It must increase much quicker, 

 or the object will break down. Every solid substance can 

 sustain a certain limited weight of its own substance, or of 

 another solid, without being crushed. The bottom bricks of a 

 wall sustain those above; the bottom layers of a tree-trunk 

 sustain those above. If the superimposed weight is too much, 

 the base gives way. These facts prescribe limits, not only to the 

 height of organized bodies, but also to their length, and that of 

 their limbs. A long earth-worm could not be constructed of 

 the soft material of many infusoria without breaking himself 

 every time he moved ; and many elegant aquatic worms are 

 able to enjoy their lives, notwithstanding the extreme delicacy 

 of their bodies, because the fluid in which they dwell sustains 

 their weight, and lessens the friction with which they glide 

 over other bodies. 



In the hydra or polyp, so common in our ponds, we notice 

 that when the creature pleases he can extend his arms many 

 times the length of his body, and still use them with effect ; 

 but if we could put out an arm twelve feet long, it would be a 

 poor implement, because muscular strength, strength of ma- 

 terial, and the conditions under which it was employed, would 

 not be in harmony with each other. 



Ciliary motion affords a beautiful illustration of mechanical 

 principles. In the first place, the shape of each cilium is such 

 as to cut readily through the water ; and, in the second place, 



