April 1, 1SG/.] 



IIARDWICKE'S SCIENCE-GOSSIP. 





quite easy to see that in the legs it is the part of 

 the appendage which corresponds with the inner 

 division, which becomes modified into what we 

 know familiarly as the "leg," while the middle 

 division disappears, and the outer division is hidden 

 under the carapace. Nor is it more difficult to dis- 

 cern that, in the appendages of the tail, the middle 

 division appears again, and the outer vanishes ; 

 while on the other hand, in the foremost jaw, the 

 so-called mandible, the inner division only is left ; 

 and, in the same way, the parts of the feelers and 

 of the eye-stalks can be identified with those of the 

 legs and jaws. 



But whither does all this tend ? To the very 

 remarkable conclusion that a unity of plan, of the 

 same kind as that discoverable in the tail or abdo- 

 men of the lobster, pervades the whole organization 

 of its skeleton, so that I can return to any one of 

 the rings of the tail, and by adding a third division 

 to each appendage, use it as a sort of scheme or 

 plan of any ring of the body. I can give names to 

 all the parts, and then if I take any segment of the 

 body of the lobster, I can point out exactly what 

 modification the general plan has undergone in that 

 particular segment ; what part has remained move- 

 able, and what has become fixed to another ; what 

 Las been excessively developed and metamorphosed, 

 and what has been suppressed. 



But I imagine 1 hear the question, how is all this 

 to be tested ? No doubt it is a pretty and ingenious 

 way of looking at the structure of any animal, but 

 is it anything more ? Does Nature acknowledge in 

 any deeper way this unity of plan we seem to 

 trace ? 



The objection suggested by these questions is a 

 very valid and important one, and morphology was 

 in an unsound state so long as it rested upon the 

 mere perception of the analogies which obtain 

 between fully formed parts. The unchecked inge- 

 nuity of speculative anatomists proved itself fully 

 competent to spin auy number of contradictory 

 hypotheses out of the same facts, and endless mor- 

 phological dreams threatened to supplant scientific 

 theory. 



Happily, however, there is a criterion of morpho- 

 logical truth, and a sure test of all homologies. 

 Our lobster has not always been what we see it ; it 

 was once an egg, a semifluid mass of yolk, not so 

 big as a pin's head, contained in a transparent 

 membrane, and exhibiting not the least trace of any 

 one of those organs whose multiplicity and com- 

 plexity, in the adult, are so surprising. After a 

 time a delicate patch of cellular membrane appeared 

 upon one face of this yolk, and that patch was the 

 foundation of the whole creature, the clay out of 

 which it would be moulded. Gradually investing 

 the yolk, it became subdivided by transverse con- 

 strictions into segments, the forerunners of the 

 rings of the body. Upon the ventral surface of 



each of the rings thus sketched out, a pair of bud- 

 like prominences made their appearance — the rudi- 

 ments of the appendages of the ring. At first all 

 the appendages were alike, but as they grew, most 

 of them became distinguished with a stem and two 

 terminal divisions, to which in the middle part of 

 the body was added a third outer division ; and it 

 was only at a later period that, by the modification 

 or abortion of certain of these primitive consti- 

 tuents, the limbs acquired their perfect form. 



Thus the study of development proves that the 

 doctrine of unity of plan is not merely a fancy ; 

 that it is not merely one way of looking at the 

 matter, but that it is the expression of deep-seated 

 natural facts. The legs and jaws of the lobster 

 may not merely be regarded as modifications of a 

 common type, — in fact and in nature they are so,' — 

 the leg and the jaw of the young animal being, at 

 first, indistinguishable. 



These are wonderful truths, the more so because 

 the zoologist finds them to be of universal applica- 

 tion. The investigation of a polype, of a snail, of 

 a fish, of a horse, or of a man, would have led us, 

 though by a less easy path, perhaps, to exactly the 

 same point. Unity of plan everywhere lies hidden 

 under the mask of diversity of structure — the com- 

 plex is everywhere evolved out of the simple. 

 Every animal has at first the form of an egg, and 

 every animal and every organic part, in reaching its 

 adult state, passes through conditions common to 

 other animals and other adult parts ; and this leads 

 me to another point. I have hitherto spoken as if 

 the lobster were alone in the world, but, as I need 

 hardly remind you, there are myriads of other 

 animal organisms. Of these, some— such as men, 

 horses, birds, fishes, snails, slugs, oysters, corals, 

 and sponges— are not in the least like the lobster. 

 But other animals, though they may differ a good 

 deal from the lobster, are yet either very like it, or 

 are like something that is like it. The Cray fish, 

 the rock lobster, and the prawn, and the shrimp, for 

 example, however different, are yet so like lobsters, 

 that a child would group them as of the lobster 

 kind, in contradistinction to snails and slugs ; and 

 these last again would form a kind by themselves, 

 in contradistinction to cows, horses, and sheep, the 

 cattle kind. 



But this spontaneous grouping into " kinds " is 

 the first essay of the human mind at classification, 

 or the calling by a common name of those things 

 that are alike, and the arranging them in such a 

 manner as best to suggest the sum of their likenesses 

 and unlikenesses to other things. 



Those kinds which include no other subdivisions 

 than the sexes, or various breeds, are called, in 

 technical language, species. The English lobster is 

 a species, our cray fish is another, our prawn is 

 another. In other countries, however, there are 

 lobsters, cray fish, and prawns very like ours, and 



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