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Show 386 EMBRYOLOGY. [CHAP. XIII. qui• t e possi' ble , that each of the many. sudc c·e ssive Inot ditf ica-tions, by which each species has acqune 1ts presen .s ruc-ture may have supervened at a not very early .peri?d of 11. ~1 e !, ana · som e di' rect evidence from our. do· mes'ttic anim'balls supports this view. Bu~ in o~her cases It IS qui e possi e that each successive modification or I?ost of them may have a eared at an extremely early period. . pp 1 have stated in the first chapter, that there IS some evidence to render it probable, that at whatever age any variation first appears in the parent, i~ tends to ~eappe.ar at a corresponding age in the offs~ring. Certa;n vanations can only appear at ~orresponding ag~s, for Instance, peculiarities in the caterpillar, cocoon, or Imago states of the silk-moth· or again in the horns of almost full-grown cattle. But further th~n this, variations :which, for ~11 that we can see, might have appe~red ear~Ier or late~ In life, tend to appear at a correspond~ng age In t~e .off~pnng and parent. I am far from meaning that th1s IS Invanably the cas~ ; and I coul~ give a good many cas~s of variations (taking the word In the_largest se?se) whiC~ have supervened at an earlier age In the child than In tha pareTnhte. se two principles, if their tru~h be a dn ~n·t te d , W'1I1., I believe, explain all the above spemfied leading facts In embryology . B~t firs~ ~et us look at a f~w analogous cases in domestic vanetles. Some autho1s who have written on Dogs, m.aintain t~at the greyhound and .bu}ldog though appearing so different, are really varieties mo~t closely allied, and have probab~y descended from the same wild stock ; hence I was curious to see how far their puppies differed from each other : I w~s told by breeders that they differed just as n1nch as then~ parents, and this, judging by the eye, seemed · almost to be t~e case; but on actually measuring the old dog:s and tbmr six-days old puppies, I found that the pupp~es had. no: nearly acquired their full amount of proportional d1ffe1- ence. So again, I was told that the foals of ca~t and racehorses differe<l as much as the full-grown animals; and this surprised me greatly, as I think it probable that the difference between these two breeds has been wholly CHAP. XIII.] EMBRYOLOGY. 887 caused by selection under domestication. but havin h careful measurements made of the d ' d f gl ad d ld 1 t f am, an o a t lreeays o co o a race and heavy cart-horse I find th t the colt~ have .by no means acquired their full am t af proportional drfference. oun o As the evi~ence appears to Ine conclusive, that the severa~ domes~IC breeds of Pigeon have descended from one wild .sp~mes, I compared young pi eons of various breeds, Within twelve hours after being~atched. I . f 11 d th . , ca1 e-u y measure e proportions (but will not h · details) of the beak, width of mouth length efre gtiv.el d f I'd . f ~ , o nos n an o . eye I , Size o 1~et and length of leg, in the wild stock, In pouters, fantails, runts, barbs, dragons, carriers, a~d tumblers. ~ow. so~e of these birds, when mature, differ so extraordmarily In length and form of beak that they would, I cannot doubt, be ranked in distinct g~nera ~ad t~ey been natural productions. But when the nest~ hng .buds . of these several breeds were placed in a row though most o.f them co~ld be distinguished from each othe~·, yet thmr pr?portlonal differences in the above specrfied scveral.points were incomparably less than in the full-grow:n birds. Some characteristic points of difference- for Instance, that of the width of mouth-could hardly be detecte~ in the . young. But there was one remarkable exception to this rule for the young of the short-~aced tumbler differed from' the young of the wild rock-pigeon and of the other breeds, in all its proportions, almost exactly as 1nuch as in the adult state. The tw~ principles above given seem to me to explain these f~cts In ;re~ard to th~ later embryonic stages of our domestic varieties. Fanciers select their horses do()'s an d pi.g eons, ~.£'o r. b ree d.1 ng, when they are nearly ' growb n' up: t~ey are 1ndrfferent whether the desired qualities and structures have been acquired earlier or later in life if the f~ll-grown animal possesses them. And the cas~s just given, more e~p~cial~y that of pigeons, seem to show that the characteristic differences which give value to each ~reed, and which have been accumulated by man's selec:; n? have not generally first appeared at an early period life, and have been inherited by the offspring at a cor- |