July 20, 188.3.] 



* KNOWLEDGE ♦ 



33 



AN 1UliS?RATED 



MAGAZINE OF S€rENCE 



PIAINLT^ORDED -£XACTL|gESCRIBED 



LONDON : FRIDAY, JULY 20, 1883. 



Contents of No. 90. 



A Naturalist's Year. XVII, Crabs 

 and Lobsters. By Grant Allen ...33 



How to Get Strong ; Reducing 

 Fat 31 



The Chemistry of Cookery. XIV. 

 By W. Mattieu Williams 33 



The Morality of Happiness : Intro- 

 ductory. By Thomae Foster 36 



The Birth and Growth of Myth. 

 XI. By Edward Clodd 37 



Laws of Briehtnesa. VI. By 

 Uichard A. Proctor 38 



Are Copper Salts Poiaonous? 39 



The Fisheries E.xhibiiion. (/«»»). 



Bv John Ernest Ady 4C 



Flight of a Vertical Missile. By 



Biehard A. Proctor 41 



Editorial Gossip 42 



The Face of the Sky. By F.E.A.S. « 



Something about the Beet. II 43 



Correspondence ; Flight of Missiles 



—Binocular Vision, &c 44 



Our Mathematical Column : Geome- 

 trical Problems. VIII 46 



Our Whist Column 47 



Our Chess Column 48 



A NATURALIST'S YEAR. 



By Grant Allen. 

 XVII.— CRABS AND LOBSTERS. 



AMONG the olive-green hanging seaweed that drops 

 like a curtain from the landward side of this broken 

 ledge of rock, I have just disentombed with my stick a 

 little soft-shelled crab, who, bent on indulging his naturally 

 retiring disposition, had taken refuge there in solitary 

 security, when the tide began to leave his open feeding- 

 grounds high and dry. See him scuttle hastily sideways 

 now across the bare patch of beach, and make his way 

 with all convenient speed towards the neighbouring pool, 

 where he proceeds at once to bury himself, tail foremost, 

 once more under the wet sand beneath the overhanging 

 eaves of the ledges ! How fast and how clumsily he runs ! 

 How quickly and yet how awkwardly he grubs his way 

 into the yielding quicksands ! A crab always suggests to 

 one the notion of an animal that has not quite fully 

 adapted itself to the conditions under which it is living. It 

 seems to have learnt the trick fairly well, so to speak, 

 but not to be able to perform it with perfect grace 

 and ease. This is a feeling one often has about 

 creatures which have widely diverged in habit, and 

 especially in attitude, from tlie rest of their kind. They 

 always seem to be clumsy in their movements. One 

 notices it, among birds, in the penguin, the puffins, and 

 many other erect species, as well as in the waddling of 

 ducks ; or among mammals, in the seals and their con- 

 geners, as well as in the kangaroos and jcrl)oas. Nay, 

 even man himself has something of the sanu^ awkwardness 

 about liim, for though he can walk well enough, he can't 

 squat or sit gracefully without the aid of an artificial 

 nieclianism, sucli as a chair or a sofa. In all these cases, 

 we may fairly say that the peculiar modilication of the 

 ancestral type for a sjiccial purpose has entailed a certain 

 newssary clumsiness in other matters, because the original 

 limbs were fitted for difl'erent ends from those to which 

 they are now applied. The clumsiness is due, in fact, to 

 what one may describe as patching and altering on the 

 part of Nature. 



The crab is an excellent example of such natural after- 



thoughts. By descent all crabs are, roughly speaking 

 lobsters ; or, to put it more correctly, both crabs and 

 loljsters are derived from a single common ancestral form, 

 from which the crab has diverged a great deal, while the 

 lobster has diverged relatively little. Crabs, in fact, may 

 be approximately described as the kind of lobsters that 

 has taken to walking instead of swimming and jumping. 

 If you compare a lobster with any one of its less developed 

 relations, such as a prawn or a shrimp, you will see that 

 while they differ in many important points of structure 

 (which I leave aside, as belonging rather to the province of 

 professional biologists, like Dr. Wilson, than of mere 

 strolling field naturalists like myself), they agree in certain 

 general proportions of the most conspicuous external 

 parts. They have all long, large muscular tails, wliich 

 contain the principal fleshy part of the body, and 

 which, in fact, we all know particularly well, because 

 they form the portion that we ourselves use as food, 

 and they have a cylindrical head and chest (so to say), 

 as well as a considerable number of swimming appen- 

 dages. But if you compare a lobster with a crab you will 

 find that, while they agree with one another in many impor- 

 tant points of structure far more closely than they do with 

 their lower relations, they yet diflPer in general outward 

 appearance far more widely than the prawn and shrimp. 

 It is these conspicuous outward differences — merely adap- 

 tive points to the true biologist — that most require expla- 

 nation in the eyes of the world at large. The other matters 

 are the really important ones — the ones on which the science 

 of biology must be fundamentally based ; but these are the 

 ones that interest ordinary people the most. 



Now, the adaptive differences between crabs and lobsters 

 are all due to the initial fact that the crab is a walking 

 crustacean, while the lobster is a jumping and swimming 

 one. If you watch the lobsters in an aquarium, you will 

 see that they hardly use their small legs for walking at all ; 

 they employ them almost entirely for standing, or rather 

 for poising themselves lightly upon the shelves of rock 

 where they love to loiter. There are two large muscular 

 tracts in the lobster's body — the two parts which we mainly 

 use for food ; one is the big and powerful tail, which is the 

 real organ of locomotion in the lobster ; the other is the 

 two great claws, which, of course, are used as organs of 

 prehension and weapons of ottence or defence. But if you 

 look closely at a crab, you will find that though its mem- 

 bers answer in the main, part for part, to the members 

 of the lobster, its shape and relative development 

 are very diflerent The tail, instead of being a 

 large fleshy organ, capable of producing long and rapid 

 springs through the water, is doubled up and tucked away 

 under the crab's body ; and, as we have frequently had 

 occasion to observe when eating cral) (the only form of 

 dissection that most of us ever practise), it contains no 

 meat worth the trouble of extracting. The fact is, most 

 crabs have so long found their tails a mere incumbrance, 

 and have consequently tucked them quietly away behind 

 them, that they have gradually dwindled by disuse, in 

 accordance with a well-known general law, that all parts 

 which are seldom or never exercised tend slowly to atrophy 

 and obsolescence. Use and disuse combine with natural 

 selection thus to alter the forms of organs. A part which 

 is much exercised tends to grow large and prominent ;. a 

 part which is little exercised tends to fade imperceptibly 

 away. 



The other clause of this law is equally well exemplified 

 in the body and legs of the crab. What most people natu- 

 rally regard as the whole creature (barring the small and 

 inconspicuous tail) answers in fact merely to the front 

 half or " body " of the lobster ; and as this part contains 



