KNOWLEDGE. 



[January 1, 1898. 



other groups. Thus the intention of the descriptive word 

 is defeated, and, instead of teaching, it leads the unwary 

 learner astray. A name like Malaeostraca, signifying 

 soft-shelled, which at one time may have usefully dis- 

 tinguished lobsters and prawns from the oyster and the 

 whelk, is no longer instructive in an enlightened age 

 which could not dream of confusing a tasteful crustacean 

 with a succulent mollusc. Moreover, some Malaeostraca 

 have very hard shells, far surpassing in induration those 

 of the Entomostraca and of many Mollusca. The name 

 Thyrostraca, meaning shells with doors or valve-shells, 

 gives a small item of information about cirripedes, while 

 the latter more familiar name refers to the 

 fact that the cirri or legs of a barnacle 

 have some resemblance to ringlets or 

 tresses of hair (see Fig. 2). None the less, 

 some of the group have no shells and no 

 valves and no cirri. 



In this opening chapter it would be 

 highly proper and methodical to define 

 the class under discussion in such a way 

 that any schoolboy, or a poet, or a 

 journalist, on coming casually across 

 a Notopteiiipliorus pupilio, for example, 

 might, under the guidance of the 

 definition, be able at once to exclaim, 

 " Lo ! here is a crustacean ! " But nature, 

 rejoicing in the penumbra and the twilight, 

 and abhorrent of every hard line, takes a 

 pleasure in setting definitions at defiance, 

 varying the characters within a group, 

 and adding here and subtracting there, 

 till there is pretty well nothing left which 

 all the confederated members can claim 

 to have in common. What if some of the 

 Crustacea are endowed with a crustaceous 

 integument : with gills for breathing ; 

 with a heart ; with eyes and brain ; with 

 segmented body and limbs ; with bilateral 

 symmetry and with powers of locomotion .' 

 There are others which are soft-skinned, without gills, eye- 

 less, brainless, heartless, shapeless creatures, in a state of 

 fixation (see Fig. 3). The difficulty of defining natural groups 

 may be illustrated in this way. Suppose that three sets 

 of animals have characters so combined that they may be 

 represented respectively by the letters nli, be, ck, or by the 

 colours red and yellow, yellow and green, green and red. 

 The symbols indicate that each set has half its characters 

 in common with each of the other sets. Yet there are no 

 characters common to all three sets, so as to be available 

 for defining a higher group embracing them all. When 

 in such circumstances a definition has to resort to negative 

 and alternative characters, it may be logically exact, but 

 it loses the quality of helpfulness. The beginner, there- 

 fore — perhaps the resentful beginner — must say what he 

 pleases, and make what he can of the statement that the 

 division of the Arthropoda called Crustacea have a seg- 

 mented body and limbs at some stage of hfe ; that either 

 they have gills or else they breathe in water through their 

 skin ; that they have no proper neck ; that they never have 

 wings ; and that they are born in locomotive freedom. 

 Like insects, they have an integument composed of a sub- 

 stance called chitine. This may be extremely flexible, or, 

 passing through various degrees of tough and brittle, may, 

 by the copious addition of chalky material, attain the hard- 

 ness of bone or brick. 



Having come to a provisional agreement with ourselves 

 that an almost indefinable congress of startlingly incon- 

 gruous-looking creatures are all to be admitted to the 



Fio. 2. — Lepas 

 anatifera (Lin- 

 DiBus), A pedun- 

 culated Cirripede. 



m 



k 



honourable title of crustaceans, we are next tempted to 

 ask what natural bond of union, if any, exists for such an 

 assemblage. Were they all separately invented just as we 

 find them, with their striking contrasts and innumerable 

 gradations and subtle resemblances ; or, have they been 

 evolved in ramifying lines from a common root ? The 

 first hypothesis would leave us rather idiotically gaping 

 at what must seem to be the eiiects of an unfathomable 

 caprice. Probably, therefore, most thinking men would 

 now prefer to explain the genesis of the " Karkinokosm," 

 as we know it, on the principle of evolution. By this 

 we mean that all the forms, now so amazingly unlike 

 one another, are nevertheless descended from common 

 ancestors. No one denies that animals are capable of 

 reproducing their kind. No one denies that children are 

 more or less unlike their parents and unlike one another. 

 That these unlikenesses can be to some extent accumulated 

 has been proved. That in the course of nature they are 

 capable of an accumulation 

 so extended and so per- 

 manent as to separate a 

 man from a mouse, or the 

 great Cardi'^dma inM»humi, 

 figured on the next page, 

 from the worm-like para- 

 site Leniaoloplnis stdta7ta,iB 

 yet awaiting proof. To the 

 principle of evolution it 

 matters not how the varia- 

 tions are produced, so long 

 as some of them can 

 sometimes be secured 

 against reversion to the 

 ancestral pattern. So far 

 as the principle is con- 

 cerned, it is indifferent 

 whether the changes result 

 in exalting or degrading 

 the character of a species. 

 To explain the existing 

 constitution of the class 

 Crustacea, it must be sup- 

 posed that some of its 

 members have risen, and 

 that some have, after 

 rising, fallen. If it cannot 



be proved that all have been evolved from a common 

 stock, something can be said for the probability of it : and 

 those who are dissatisfied can only be asked to provide 

 some other explanation that will better fit the phenomena. 

 For the purposes of a natural classification it is the 

 history of evolution that is most wanted. We need to 

 trace back the ancestry of different forms to the point of 

 junction, just as we foUow the twigs of a tree to the 

 branch from which they spring, and the branches to the 

 common stem. Clearly this can only be done by help of 

 the palaeontologists. What the rocks have as yet revealed 

 as to the succession in time of crustacean forms has 

 recently been represented by Dr. Henry Woodward in a 

 kind of fossil tree. Of the undisputed Crustacea he 

 recognizes eleven principal branches, and all these he 

 draws as running parallel down to the Carboniferous 

 period — a period so ancient that in calculating its age 

 imagination and arithmetic have to play a drawn game, and 

 yet so modern that in it the merry cockroach is already 

 in evidence. The disappointing inference is that any 



* See his Presidential Addresses to the Geological Society of 

 London, 1895, 1896. 



Fig. 3. — Lernteolophus sultana 

 (Xordmann). A Copepod, parasitic 



on Fisli. 



