Principles of comparative physiology / by William B. Carpenter.

  • William Benjamin Carpenter
Date:
1854
Catalogue details

Licence: Public Domain Mark

Credit: Principles of comparative physiology / by William B. Carpenter. Source: Wellcome Collection.

Provider: This material has been provided by Royal College of Physicians, London. The original may be consulted at Royal College of Physicians, London.

    130/810 (page 96)
    genus, species, sex, and individual, in succession.* This, at least, is the result of observations made in a considei-able number of cases; and where such an accordance does not exist, the want of it is probably due to im- perfections in the system of classification with wliich the comparison is made.—Thus we see that in watching the history of the development of any one of the higher forms of organised structure, we find the realization of that ideal evolution of the more s]}ecial characters from the more general, which it is the object of the Philosophic Natviralist to bring into view by the methods of proceeding already pointed out (§ 11). 74. The general principle of Yon Baer aflfiords the real explanation of those resemblances which are sometimes discernible, between the ti-ansitory forms exhibited by the embryoes of higher beings, and the permanent conditions of the lower. When these resemblances were first observed in the stiidy of Embryology, an attempt was made to generalize them in the statement that the higher animals, in the progress of theii development, pass through a series of forms corresponding with those that remain perma- nent in the lower parts of the animal scale. But this statement was hasty and unphilosophical; and it is now only referred-to, for the sake of show- ing what amount of real truth there is in it.—No animal as a whole passes through any such series of changes, except where it comes forth from the egg in an early stage of development, but in a condition that enables it to sustain its own existence, and to lead the life of a class below, from which it is afterwards raised by metamorphosis. This is the case, for example, with such Insects as resemble Annelida in their larva condition, and with Batrachian ReptUes, which are essentially Fish dming the early period of their lives. But in neither of these instances, does the Larva entirely resemble the perfect animal which it represents in form and gi'ade of organisation; for besides having its generative system undeveloped (with- out which it cannot be said to be a complete animal), the condition of its tissues and organs is altogether embryonic; so that the caterpillar bears a much closer accordance with the embryonic than with the adult Annelide, while the Tadpole is more nearly related to the embryonic than to the perfected Fish. These and other cases of the same kind must be regarded as special modifications of the general plan to meet a particular purpose; and while they present nothing discordant with that plan, they cannot be taken as examj^les of the usual mode in which it is followed out. On studying the development of any one of the higher animals, which remains within the ovimi until it has attained the form character- istic of its class, we find that its entire structure does not present at any time such a resemblance to either of the classes beneath, as would * Althougli this general truth had been previously indicated hy Von Baer, yet the first definite and complete statement of it, with its appUcation to Classification, wiU be found (the Author believes) in two papers ' On Unity of Structure in the Animal Kingdom' con- tributed by Dr. Martin BaiTy to the Edinburgh Philosopliical Journal for 1837. It has been subsequently developed in a very admii-able manner by Prof. Mike-Edwards in a Memoir ' On the Principles of the Natural Classification of Animals, m the Anuales des Sciences Naturelles, Ser. in., tom. i., which bears evidence of having been written with- out the knowledge of what either Von Baer or Dr. Barry had put forth; the pi-mciple, in fact, having been advanced in a more limited form by Prof. Milne-Edwards himself, in a memoir ' On the Changes of Form exhibited by various Crustacea during their Develop- ment,' read by him to the French Academy in 1833, and pubbshed in the Annales des Sciences Naturelles, Ser. i., tom. xxx., Ser. ii., tom. m.