12 THE MECHANISM OF LIFE 



cutting movements of the mandible, and pincers in crabs and 

 lobsters; piercing and sucking by the proboscis of such an insect 

 as a mosquito; stinging by bees and wasps, aided by the injection 

 of poisons; the gripping and sucking action of the cuttlefish, 

 squid, or starfish; snaring movements involving constructive 

 devices, as in the web of a spider, and so on. All these are just 

 as characteristic of groups of animals as are the locomotory 

 patterns. 



Such mobility patterns go roughly side by side with structure 

 patterns; thus, quadrupedal locomotion involves the use of fore 

 and hind limbs; flight that of one or two pairs of wings; swim- 

 ming means that there are fins (or other movable and fixed 

 hydroplanes), or cilia, or flagella ; creeping requires a number of 

 serially arranged appendages (like the " legs " of a centipede, or 

 the setse of a worm), and so on. But the parallelism between 

 structure and mobility patterns is not always complete; thus 

 the dorsal fin of a fish may be a vertical rudder, a poison organ, 

 or a lure; certain bones in the head of a vertebrate animal may 

 be parts of the skeleton of the jaws (in fishes), or the little " audi- 

 tory ossicles" of the internal ear (in mammals); the same 

 (historical) limb may be a paddle (in a seal), a wing (in birds), 

 or an arm and hand (in man). Knowing what the structure is 

 does not always mean that we can deduce the function, and vice 

 versa. 



And we are mainly interested in either the structure or the 

 function of the structure, according to our point of view. The 

 former is our main object of study from an historical aspect, as 

 when the same structure gradually became modified; thus a 

 seal is a modified carnivore which has adopted an aquatic habit, 

 in the course of which a typical quadrupedal limb became 

 modified in structure to serve as a paddle. But here we are 

 concerned with an animal mechanism, and so we regard paddles, 

 fins, and limbs as three variants of a generalised locomotory 

 pattern. 



That means that it is the animal as a whole that we are study- 

 ing: its modes of movement that enable it to distribute itself, 

 to find shelter, to escape from its enemies, to capture and kill 

 the organisms which serve as its food — in general, its behaviour 

 and the ways in which this is expressed. And so we must study 

 the structure of the mobile animal parts as a means of our 

 analysis of living activity. 



