OCR Text |
Show 396 SUMMARY. (CHAP. XIII. might even have been anticipated, and can be accounted for by the laws of inheritance. Summary.-In this chapter I have att~mpted to s~ow, that the subordination of group to group In all organisms throughout all. t~me ; that t~e nat~re of the ~·elationship, by which all hv1ng all:d e~t1nct ~mngs are UI_n~ed _by colnplex radiating, and cn·c1ntous hnes of affinities Into one gran'd systmn ; the rules followed and the difficulties encountered by naturalists in their classifications; the value set upon characters, if constant and prevalent, whether of high vital importance, or of the most trifling importance, or, as in rudimentary organs, of no importance ; the wide opposition in value between analogical or adaptive characters, and the characters of true affinity; and other such rules ;-all naturally follow on the view of the common parentage of those forn1s which are considered by naturalists as allied, together with their modification through natural selection, with its contingencies of extinction and divergence of character. In considering this view of classification, it should be borne in mind that the element of descent has been universally used in ranking together the sexes, ages, and acknowledged varieties of the sam·e species, however different they may be in structure. If we extend the use of this element of descent,the only certainly known cause of similarity in organic beings, we shall understand what is meant by the natural system : it is genealogical in its attempted arrangement, with the grades of acquired difference marked by the terms varieties, species, genera, families, orders, and classes. On this same view of descent with modification, all the great facts in Morphology become intelligible,whether we look to the same pattern displayed in the homologous organs, to whatever purpose applied, of the different species of a class ; or to the homologous parts constructed on the same pattern in each individual aniInal and plant. On the principle of successive slight variations, not necessarily or generally supervening at a very early period CRAP. XIII.] SUMMARY. 397 of life, and being inherited at a corresponding period we can understand the grea~ leadi~g _facts in Embryol~g ; namely, the resemblanc~ In an Individual embryo of the h~molog~us parts, whiCh when matured will become '!Idely different from each other in structure and functiOn ; and the resemblance in different species of a class of the homologous parts or organs, though fitted in the adult membe.rs for purposes. as different as possible. Lar':re are. active ~mbryos, whiCh have become specially m~di~ed In rel~tion. to th~ir ~a bit~ of life, through the prmmple of n;odificatio~s ~mng Inherited at corresponding ages. On this same pnn~Ipl~-an~ bearing in mind, that when organs are reduced IU size, either from disuse or sele~ tion, it will gen~rally b~ at that period of life when the b~Ing has to pro;Ide for ~ts _own wants, and bearing in mind how strong IS the prinCiple of jnheritance-the occurrence of rudimentary organs and· their final abortion pre~ent to us no inexplicable difficulties ; on the contrary: ~hmr presence might haye been even anticipated. The nnportance of embryological characters and of rudimentary organs in classification is intelligible, on the view that an arrang-ement is only so far natural as it is genealogical. Finally, the several classes of fac.ts which have been considered in this chapter, seem to me to proclaim so plainly, that the innumerable species, genera, and families of organic beings, with which this world is peopled, have all descended, each within its own class or group, from common parents, and have all been modified in the course of descent, that I should without hesitation adopt this view, even if it were unsupported by other facts or arguments. |