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Show 104 HYPOTHESIS 011' THE DEVELOPMENT OP and mammalia. This is another advance in the scaie, but more remains yet to be done. The complication of the organ increases; cavities termed vent·ricles are formed. which do not exist in fishes, reptiles, or birds; curiously organized parts, l'lUch as the. corpora striata, are added ; it Is now the brain of the mammalia. Its last and fina] change alone seems wanting, that wl>;ich. shall render it the brain of MAN."* And th1s change In time takes place. So also with the heart. This organ, in the marnmalia, consists of four cavities, but in the reptiles of only three, and in fishes of two only, while in the articulated animals it is merely a prolonged tube. Now in the mammal fcetus, dt a certain early stage, the organ ha3 the form of a prolonged tube; and a human being may be said to have then the heart of an insect. Subsequently it is shortened and widened, and becomes divided by a contraction into two parts a ventricle and an auricle; it is now the heart of a fish. A subdivision of the auricle afterwards n1akes a triple chambered form, as in the heart of the reptile tribes; laslly, the ventncle being also subdivided, it becomes a full mamma1 beart. Another illustration here presents itself with the force of the most powerful and interesting analogy. Some of the earliest fishes of our globe, those of the Old Red Sandstoue, present, as we have seen, certain peculiarities, as the one-sided tail and an inferior position of the mouth. No fishes of the present day, in a mature state, are so characterized; but some, at a certain stage of their existence, have such peculiarities It occurred to a geologist to inquire if the fish which existed before the Old Red SandstonA had any peculiarities assimilating them to the fretal condition of existing fish, and particularly if they were small. The first which occur before the time of the Old Red Sandstone, are those described by Mr. Murchison, as belonging· to the Upper Ludlow Rocks; they are all rather small. Still older are those detected by Mr. Philips, in the Aymestsy Limestone, being the most ancient of the class which have as yet been discovered; these are so extremely m,inute as only to be dt'stinguishable by tht microscozJe. Here we apparently have very clear demon· strations of a parity, or rather identity, of laws presiding " Lord's Popular Physiology. It is to Tiedemann that we chief· ly owe these curious observations- but ground lvas first broken lD this branch ofphysiologicul science by Dr. John Hunter Tl-IE VEGETABLE A 'WD ANIMAL KINGDOMS'. 105 ever the development of the animated tribes on the face of the earth, and that of the indi~idual i~ embryo. The tendency of all these Illustrations is to make us look. to de·velopnzent ~s the principle which has been im- 1ned1ately c~mcerned lL the peopling of this globe, a process extend1ng o.ver a v~st space of time, but which is nev~rthele~s. conl?ec~e? 1n cha~·acter with the briefer process by wh1cn. an IndiVIdual being is evoked from a simple germ. 1Vhat ~ystery is there here-and how shall I proceed to enunciate the conception ·which I have ventured ~o for~ of what may prove to be its proper solution! It 1s an Idea py no mea~s calculated ·to impress by its greatness, or to p~zzle. by Its profoundness. It is an idea more rna~· ked by simpli?Ity . than perhaps any other of those W~I~~ have explained the great secrets of nature. But in this 11es,.perhaps, one of its strongest claims to the faith lf manlund. The whole train of _animated beings, ·from the simplest and oldest up to the hLghest and most recent are then to be regarded as .a series of advances of the 11rinci;le of de.,~ loyment, w h1eh ha~e depended upon external physical CL~:cumstances, to wh1ch the resulting animals are appro-pnate.- I contemplate the whole phenomena as having be~n In the first place arranged in the counsels of Divine WJsd.)m, to take pl~ce not only upon thi~ sphere, but ~pon all the ot~ers In .space, under necessary modifica-tions. and as being ca:ned o?, frDm first to last, here and elsewhere, under the Immediate favor of the creative ·will or energy.*. Th.e nucleated vesicle, the fundamental form of all organiza~wn, w~ must regard as the meeting-point he.tween the InorganiC and the organic-the end of the ~Ineral and t~e beginning of the vegetable and animal ~ungdoms, whiCh thence start in different directions but In perf~ct parallelisll} and analogy. We have already ~een ~hat., ~his nuclea.ted .vesicl~ is itself a type of mature and Indt: fen~ent b~mg In the Infusory animalcules, as well as tl~e.s .arh~g po1n~ of the fcetal progress of every higher in .. divLdualin creation, both animal and vegetable. We have fa~ntf~en / for~edd thi.s idea, I was not aware of one which seems A J,. /){j . dilat~ :n ~;e~l:too~h l~~name~y, Socrates's .doctrine, afterwards d b . ' a prevwus to the existence of the world :n b ~rondtit(~present limits, there existed certain archetypeA th~ t/fl t ~I noen hJ we may use such a word) of g~neral ideas·' and la Jes.e arc etypes were models .. in imitation of w h · h 11 ' rt'· cular bemgs were created.,, IC a pa 1 g |
