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Show 372 CLASSIFICATION. [CHAP. XIII. descent we can clearly understand why .analogical or adaptiv~ character, although of the utmost Importance to the welfare of the being, are al:nost valueless to th.e ~ystmnatist. :For animals, belonging to two most d~sti;l.Ct lines of descent, may readily become adapted to Similar d·t· ns and thus assume a close external resmnblance; bcount sI uIcOh 'r esemblances will not. revea1 - .WI'1 1 rat h ~r t en d to conceal their blood-relationship to thmr proper hnes of descent. We can also understand the appa;ent paradox, that the very same character~ are analogical w~en one class or order is compared With another, but give true affinities when the members of the same class or order are cmnpared one ·with another : thus t~e shape of the body and :fin-like limbs are only analogl.C~l w~en whales are coinpared with :fishes, being adaptations 1n both classes for swimming through the water ; but the sl!ap~ of the body and :fin-like lhnbs serve as characters exhibiting ~rue affinity between the several members of the whale fam1ly; for these cetaceans agree in so many characters, great and small that we cannot doubt that they have inherited their gene;al shape of body and structure of limbs from a common ancestor. So it is with :fishes. As mmnbers of distinct classes have often been adapted by successiv~ slight mod~:ficati.ons to .live under nearly similar circuinstances -to Inhabit for Instance the three elements of land air" and water,-we can perhaps understand ho'v it is that' a numerical parallelism ~as .so!netimes been observed between the sub-groups In distinct classes. A naturalist struek by a parallelisrn of this nature in any one cla~s, by arbitrarily rai.sing or sinking the value of the grou~s in oth.er classes. (and all our e~perience shows that this valuation has h~therto been ar?Itrary) could easily extend the parallelism over a Wide range'; and thus ~he septenary, quinar:y, quaternary, and ternary classifications have probably ari~en. . As the modified descendants of dominant spemes, belonging to the larger genera, tend ~o inherit the advantages, which made the groups to which they belong large and their parents dominant, they are almost sure ~o spread widely, and to seize on more and more places In OHAP. XIII.] CLASSIFICATION. the economy of nature. TJ:e larg~r and ·more dominant groups thus tend to go on Increasing in size; and the consequently supplant many smaller and feebler rou l Thus we can account for the fact that all orgg · p recen t a~ d ex tI. nc t , are I· ncluded :rnder a few great aonr1dsemrss,, 373 under still fewm: classes, and all In one great natural sys. tern. As showing .how fe·w· the higher groups are in number, and h~w w~d~ly spread they are throughout the world, the fact Is ~tnlnn.g, that the discovery of Australia has not added a single Insect belonging to a new order . and that . in the vegetable kingdom, as I learn from Dr~ ~ooker, It has added only two or three orders of small SIZe. In the chapt~r o;t geological succession I attempted to s~ow, on the pr~nmple of each group having generally diverged much. In ?haracter. d~ring the long-continued process of ~odinc!ttion, how It Is that the more ancient forms ?f hfe often present characters in some slight degr~e Inter~ediate between existing groups. A few old aD;d Intermediate parent-forms having occasionally transm~ tted. to the present day descendants but little modi£.ed, will give to us our so-called osculant or aberrant groups. The more aberrant. any form is, the greater must be the number of ?onnecting forins which on my theory have be~n exterminated and utterly lost. And we have some evi~en~e of aberrant forms having suffered severely from extinction, .for they are generally represented by extremely few ~p~mes ; and such species as do occur are generally ~ery distinct from e.ac~ other, which again impli~s extinctiOn. The genera Ornithorhynchus and Lepidosiren for example, would not have been less aberrant had ~ach bee~ represente~ by a d?zen species instead of by a single one ? b~t such nchness 1n species, as I £.nd after some investigation, does not commonly fall to the lot of aberrant fen~ra. We can, I think, account for this fact only by ooking at aberrant forms as failing groups conquered by more successful competitors, with. a few members pre~ sstearnvceeds .b y some unusual coincidence of favourable circumMr. Waterhouse has remarked that, when a member ,, |