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ANATOMY AND PHYSIOLOGY OF 



upon the properties or endowments of the 

 substances through which it acts, namely, 

 the germ on the one hand, the alimentary 

 materials on the other, — just as an electric 

 current, transmitted through the different 

 nerves of sense, produces the sensory im- 

 pressions which are characteristic of each 

 respectively ; or, as the same current trans- 

 mitted through one form of inorganic 

 matter produces light and heat, through 

 another, chemical change, or through an- 

 other, magnetism. 



" In the development of any living being, 

 therefore, from its primordial germ, we have 

 three sets of conditions to study — namely, 

 first, the physical forces which are in opera- 

 tion ; second, the properties of the germ, 

 which these forces call inta activity; and 

 third, the properties of the alimentary mate- 

 rials which are incorporated in the organism 

 during its development. There is evidence 

 that each of these may have a considerable 

 influence on the result ; but in the higher 

 organisms it would seem that the second is 

 more dominant than it is in the lower. For 

 among many of the lower tribes, both of 

 plants and animals, there is reason to be- 

 lieve that the range of departure from the 

 characters of its parent, which the organism 

 may present, is considerably greater than 

 that of the higher ; and that this is chiefly 

 due to the external conditions under which 

 it has been developed. The forms of a 

 number of species of the lower Fungi, for 

 example, appear to be in a great part de- 

 pendent on the nature of their aliment ; so 

 among the Entozoa, there seems strong 

 reason to believe that. those of the Cystic 

 order are only Cestoidea, that are prevented 

 by the circumstances under which they exist 

 from attaining their full development ; and 

 the production of a fertile 'queen' or of an 

 imperfect 'worker,' among the hive-bees, 

 appears to be entirely determined by the 

 food with which the larva is supplied. No 

 such variations have been observed among 

 the higher classes ; in which it would seem 

 as if the form attained by each germ is 

 more rigidly determined by its own endow- 

 ments ; a modification in the other con- 



ditions, which in the lower tribes would 

 considerably affect the result, being in them 

 unproductive of any corresponding change. 

 For, if such modification be considerable, 

 the organism is unable to adapt itself to it, 

 and consequently cither perishes or is imper- 

 fectly developed ; whilst, if it be less potent, 

 it produces no obvious eftbct. Thus, a de- 

 ficiency of food in the growing state of the 

 higher animal will necessarily prevent the 

 attainment of the fuU size ; but it will not 

 exert that influence on the relative develop- 

 ment of different parts that it does among 

 plants, in which it favors the production of 

 flowers and fruit in place of leaves, or that 

 it seems to exercise in several parallel cases 

 among animals. So, again, a deficiency of 

 heat may slightly retard the development of 

 the chick ; but, if the egg be allowed to re- 

 main long without the requisite warmth, 

 the embryo dies, instead of passing into a 

 state of inactivity, like that of reptiles or 

 insects. The extent, indeed, to which these 

 external conditions may affect the develop- 

 ment of the inferior organisms, must not be 

 in the least judged of by that to which 

 their operation is restricted in the higher ; 

 and it is probable that we have yet much to 

 learn on the subject. At present, it may 

 be stated as a problem for determination, 

 whether, from a being of superior organiza- 

 zation, loioer forms of living structure, 

 capable of maintaining an independent 

 existence, and of propagating their kind, 

 can ever originate, by an imperfect action 

 of its formative powers. Various morbid 

 growths, such as cancer cells, to which the 

 higher organisms are liable, have been 

 looked upon in this light ; these have cer- 

 tainly a powerful vitality of their own, 

 which enables them to increase and multi- 

 ply at the expense of the organism which 

 they infest ; and they have also an energetic 

 reproductive power, by which they can pro- 

 pagate their kind, so as to transmit the 

 disease to other organisms, or to remote 

 parts of the same organism ; but such 

 grow'ths are not independent ; they cannot 

 maintain their own existence, when de- 

 tached from the organism in which they are 



