OCR Text |
Show 388 EMBRYOLOGY. [OBAP. XIII. responding not early period. But the case of the s~ortfaced tumbler which when twelve hours old had acqurred its proper proportions, proves that this is not the universal rule · for here the characteristic differences must either hav~ appeared at an earlier period ~han :usual, or, if not so the differences must have been inherited, not at the co'r responding, but at an ear1 1' er age. Now let us apply these facts and the above two principles- which latter, though not proved true, can be shown to be in some degree probable-to species in a state of nature. Let us take a genus of birds, descended on my theory from some one parent-species, and of which the several new species have become modified through natural selection in accordance with their diverse habits. Then, from the 1nany slight successive steps of variation having supervened at a rather late age, and having been inherited at a corresponding age, the young of the new species of our supposed genus will manifestly tend to resemble each other much more closely than do the adults, just as we have seen in the case of pigeons. We may extend this view to whole families or even classes. The fore-limbs, fof instance, which served as legs in the parentspecies, may become, by a long course of modification, adapted in one descendant to act as hands, in another as paddles, in another as wings; and on the above two principles- namely of each successive modificaHon supervening at a rather late age, and bein~ inlierited at a corresponding late age-the fore-limbs 1n the embryos of the several descendants of the parent· species will still resemble each other closely, for they will not have been modified. But in each individual new species, the embryonic forelimbs will differ greatly fro1n the fore-limbs in the mature animal ; the limbs in the latter having undergone much modification at a rather late period of life, and having thus been converted into hands, or paddles, or wings. Whatever influence long-continued exercise or use on the one hand, and disuse on the other, may have in modifying an organ) such influence will mainly affect the mature animal, which has come to its full powers of activity and has to gain its own living; and the effects thus produced OBAP. XIII.] EMBRYOLOGY. 'll'b . h . 389 Wl e In ented at a corres . as the young will remain u~ond~n.g Inature age. Where-lesser degree, by the effects r;:odificd, or. he modified in a In certain cases the sue 0 • use and disuse. supervene, from causes of ~h~~he steps. of variati?n might at a very early period of 1'£ we ai e wholly Ignorant herited at an earlier eri~d' or each step might be in~ appeared. In either ca~e (as wf~a~ th~ ~t which it first the young or embryo would cl 1 e s o1 t-faced tumbler) parent-form. )\T e have seen ti~!~ Yh ~es~mble the mature velopment in certain whole rou t Is IS ~he rule of decuttle- fish and spiders and wg_ th ps :D of animals, as with . great class of insects,' as with 1 A h. ew m~mbers of the the final cause of the youn in th P IS. WIth respect to any metamorphosis, or clgsel r:se cas~s not u?-dergoing from their earliest age ,ve ca~ esotfbhnhg_ thmr parents from the two follo·win'O' conti.I1 see .lat tfiis would result young, d un.n g a cours5e of mogde.n :fci 1es . : rst.l y? fI. om th e many generations, having to prov~d~a~IOI~hc~~rled on for at ~ very early stage of develo m or en own wants their following exactly the sa.Je hn~ ·fndf sl~~ond_ly, from parents; for in this case it would ba .I ~-o I e With their existence of the species, that the ch ·~Jn hispensable fo~ the ~t a very early age in the same I s o~ld ?e modified m accordance with their simi, mhu~e_r With Its parents, explanation, however of the e la~. a Its. Some. further metamorphosis is ~rba s rem _I"{O not undergoing any hand, it profited th~ youJg to fo~isitel {!' on ~he. other degree different from those of t"h;[r la I.ts of hfe In any quently to be constructed in a 1' 1 1 pai_ent, and consethen, on the principle of inh s ·~g lt y different manner, ages, the active youn or larv erJ ~nee at. corresponding by natural selection ~i.fferent ~o U::fht easil:f be rendered from their parents. Such d 'ffi y c_onceivable extent thrrelated with successive st~g~:e~feJ IDifht, also, become 1 e laryre, in the first stage might d ·fiY~ opment; so that arvre In the second sta e ' I ei greatly from the with cirripedes. The ~ 'Its "'":ehhave seen to be the case or habits, in which or a u f ~Ig t b~come fitted for sites &o., would be uselessg.ans o . oco~otlon or of the senses, morphosis would be saidatndbin this case the final meta-o e retrograde. |