OCR Text |
Show , 102 HYPOTHESIS OF THE DEVELOPMENT OF mmnmifers; and even the serpent tribes, vrhich present no external appearance of such extremities, possess them in reality, but in an undeveloped or rudimental state. The same law of development presides over the veget~ble kingdom. A.mongst phanerogamous plants, ~ cer~a1n number of organs appear to be always present, e1t~er In a developed or rudimentary state; and those whiCh are t·udimt:>ntary can be developed by cultivation. The flowers, 'vhich bear stamens on one stalk and pistils on another can be caused to produce both, or to become perfect flowers, by having a sufficiency of nourishment supplied to them. So also, where a special function is required for particular circumstances, nature has provided for it not by a new organ, but by a modification of a common one, which she has effected in develop1nent. Thus, 'for instance, some plants destined to live in arid situations, l'equire to have a store of water which they may slowly absorb. The need is arranged for by a cup-like expansion round the stalk, in which water remains after a sho·wer. Now the pitche1·, as this is called, is not a new organ, but simply a metamorphose of a leaf. • These facts clearly show how aH the various organic forms of our world are bound up in one-how a fundamen~ al unity pervades and embraces them all, collecting them, from the humblest lichen up to the highest mammifer, in one system, the whole creation of which must have depended upon one l:nv or decree of the Almighty, though it did not all come forth at one time. After what we ha,ve seen, the idea of a separate exertion for each must appear totally inadmissible. The single fact of abortive or rudi .. rneutary organs condemns it; for these, on such a supposition, could be regarded in no other light than as blemishes or blunders-the thing of all others most irreconcj}ablc with that idea of Almighty Perfection which a general view of nature so irrcsitibly conveys. On the other hand, when the organic creation is admitted to have been effected by a general law, \Ve see nothing in these abortive parts but harmless peculiarities of development, and interesting evidences of the manner in which the Divine Author has been pleased to work. We have yet to advert to the most interesting class of facts connected with the laws of organic development. It is only in recent times that physiologists have observed that each animal passes, in the course of its germinal his- THE VEGETABLE AND ANIMAL KINGDOMS. 103 tory, through a s~ries of changes resembling th• permanent forms of the varwus order::; of animals inferior to it in the scale. Thus, for instance, an insect, standing at the head of ~he articulated anirnalR, is, in the larva ~~ ;, tc, a true a.nnelid, or worm, the annelida being the lu\' ,·st in the same class. The cmbyro of·a crab resembles the perfect tnimal of the inferior order myriapoda, and pusses through Rll the forms of transition which characterize the interme-diate tribes of crustacea. The frog, for some time after its birth, is a fish with external gills, and other organs fitting it for an aquatic life, all of which are changed as it advances to maturity, and becomes a land animal. The mammifet· only passes through still more stages, accord-ing to its higher place in the scale. Nor is man himself exempt from this law. Hjs first form is that which is permanent in the animalcule. fiis organization gradually passes through condj tions generally n~sembling a fish a reptile, a bird, and the lower mammalia, before it attains / its specific maturity. At one of the last stages of his freta! career, he exhibits an intermaxillary bone, which is characteristic of the perfect ape ; this is suppressed, and he may then be said to take leave of the simial type, and become a true human creature. Even, as we shall see, the varieties of hjs race are represented in the progressive development of an individual of the highest, before we see the adult Caucasian, the highest point yet attained in the animal scale. To come to particular points of the organization. The brain of man, which exceeds that of all other animals in complexity of organization. and fulness of development; is, at one early period, only" a simple fold of nervous matter, with difficulty distinguishable into three parts, while a little tail-like prolongation towards the hinder parts, and which had been the first to appear, is the only representation of a spinal marrow. Now, in this state it perfectly resembles the brain of an adult fish, thus assuming in, transitu the form that in the fish is permanent. In a short time, however, the structure is become mol'e complex, the parts more distinct, the spinal marrow better marked; it is now the brain of a reptile. The change continues; by a singular motion, certain parts, (corpora quad7·agemina) which had hitherto appeared on the upper surface now pass towards the lower; the former is their pennanent situation in fishes and reptilPs, the la.ttcr in bird~ |