July 25, 1878] 



NATURE 



ZZZ 



2. Separate, narrow, solid supports, in longitudinal 

 series, with their long axis at right angles with the long 

 axis of the body, were developed in varying extent in all 

 these four longitudinal folds. 



3. The longitudinal folds become internjpted variously, 

 the lateral folds so as to form two prominences on each 

 side the primitive paired limbs. 



4. Each anterior paired limb increased in size more 

 rapidly than the posterior limb. 



5. The bases of the cartilaginous supports coalesced as 

 was needed according to the respective practical needs of 

 the difterent separate portions of the longitudinal folds, 

 i.e., the respective needs of the several fins. 



6. Occasionally the dorsal rays coalesced proximally 

 and sought centripetally adhesion to the skeletal axis. 



7. The rays of the hinder paired limbs did so more con- 

 stantly, and ultimately prolonged themselves inwards by 

 mediad growths from their coalesced base till the piscine 

 pelvic structures arose. 



8. The pectoral rays with increasing development also 

 coalesced proximally, and thence prolonging themselves 

 inwards to seek a point d^apptii, shot dorsad and ventrad 

 to obtain a firm support, and at the same time to avoid 

 the visceral cavity; thus they came, to abut dorsally 

 against the axial skeleton and to meet ventrally together 

 in the middle line below. 



9. The lateral fins, as they were applied to support the 

 body on the ground, became elongated, segmented, and 

 narrowed. 



10. The distal end of the incipient cheiropterj'gium 

 either preserved and enlarged pre-existing cartilages or 

 developed fresh ones to serve fresh needs, and so grew 

 into the developed cheiropterygium. 



11. The pelvic limb acquired a soHd connection with 

 the axial skeleton — a pelvic girdle — through its need of a 

 point d^apptd as a locomotive organ on land. 



12. The pelvic limb became also elongated, and when 

 its function was quite similar to that of the pectoral limb 

 its structure also became quite similar. It became seg- 

 mented in a way generally parallel with the segmentation 

 of the pectoral limb, yet in part inflected inversely owing 

 to its different mode of use. 



Vertebrate limbs then are specialised differentiations of 

 primitively continuous lateral folds, and might, for all we 

 see, have been more numerous than two on each side, just 

 as there are sometimes several successive dorsal fins which 

 are all differentiations of a primitively continuous dorsal 

 fold. The paired limbs and azygos fins may thus be all 

 viewed as different species of one fundamental set of parts, 

 pterygia, the sum total of which may be spoken of as the 

 sympterygiiim. The paired fins of fishes are related to 

 the limbs of higher vertebrates as structures which have 

 diverged from their primitive condition to a less degree, 

 not only because the piscine body is, as a whole, a more 

 primitive structure, but also because their fins are still 

 used for locomotion in that medium in which their 

 primeval form — the continuous lateral fold — was first 

 developed. 



The amount of adaptive modification supposed will 

 perhaps appear to some persons to be excessive. But I 

 believe that the excessive plasticity of animal organisms is 

 in general too little appreciated — a plasticity which results 

 in, and is evidenced by, the many instances of homoplasy 

 — the independent origin of similar structures. The exist- 

 ence of these adaptive modifications points to the existence 

 of an intra-organic activity, the laws of which have yet to be 

 investigated. The instances of serial and bilateral homology 

 before cited from comparative anatomy, pathology, and 

 teratology, also concur in pointing to an intra-organic 

 activity, the laws of which are as yet unknown. The 

 notion of an " internal force " is very repugnant to some 

 of my contemporaries, but it is impossible to banish the 

 idea of innate powers and tendencies, the existence of 

 which is manifested in the inorganic world as well as in 



I 



the organic world. We cannot conceive the universe as 

 consisting of atoms acted on indeed by external forces, 

 but having no internal power of response to such actions ; 

 and in " physiological units " and " gemmules " we have 

 (as Mr. Lewes has remarked) " given as an explanation 

 that very power which was pronounced mysterious in 

 larger organisms." 



Mr. Lankester ^ speaks of each animal function, even 

 reproduction, as being "explained by its chemical and 

 physical constitution," and of "the possibility of de- 

 velopment" being "solely due to the physico-chemical 

 constitution of protoplasm;" but he does not give the 

 explanation, nor show how such constitution by itself 

 gives developmental power. But even if he did the 

 puzzle would but recur — By what process of the survival 

 of the fittest did the inorganic substances obtain their 

 various structures and innate powers ? 



To my mind the presence of a special internal force is 

 made evident by the process of development ; and I am 

 disposed to concur with Milne-Edwards when he says : 

 "Dans I'organisme tout semble calculi en vue d'un re- 

 sultat determine, et I'harmonie des parties ne rdsulte pas 

 de r influence qu'elles peuvent exercer les unes sur les 

 autres, mais de leur co-ordination sous 1' empire d'une 

 puissance commune, d'un plan precongu, d'une force 

 pr^existante." 



Science, as I understand it, clearly points to the 

 existence in each animal of something more than an 

 amalgam of physical forces, to a force or principle which 

 is itttfa-organic, as heat is in red-hot iron or light in the 

 glowing photosphere of the sun — one with it as the im- 

 press on stamped wax is one with the material bearing 

 such impress, though we can ideally distinguish the two. 

 This power or force immanent in each living body, 

 or rather which is the force of the body living (consi- 

 dered in an abstract way), is of course unimaginable by 

 us, since we cannot by imagination transcend experience ; 

 nothing can be imagined by us which has not wholly or in 

 its parts been the subject of our sensible experience, and 

 we can have no sensible experience of this force, save as 

 a living body acting. 



It is on this account sometimes thought reasonable to 

 deny its existence as a "figment of the intellect," for- 

 getting the supremacy of the intellect o\xr sense. Though 

 no knowledge is possible to us except as following upon 

 sensation, yet the ground of all developed knowledge is 

 not sensational, but intellectual ; it reposes ultimately 

 not on "feelings," but on thoughts. Even in verification 

 by sensation it is the intellect which doubts, criticises, and 

 judges the action and suggestions of the senses and 

 imagination. If then we have rational grounds for the 

 acceptance of such a purely intellectual conception, the 

 poverty of our powers of imagination should be no bar to 

 its acceptance. We are continually employing concep- 

 tions of the kind — such, e.g., as number, being, substance, 

 causes, &c., — conceptions perfectly intelligible, though 

 transcending the powers of the imagination. 



If, then, we should conclude that each living animal 

 possesses a special and peculiar intraorganic force, and if 

 such force be the imminent cause of nutritional balancings, 

 and thereby of the facts of serial and bilateral symmetiy,is it 

 not reasonable to refer to that same cause directly adaptive 

 modifications which, within limits, take place in response 

 to the actions of the environment. The presence of such 

 innate activity has been eloquently proclaimed by Hart- 

 man, though I would repudiate the contradictory term 

 " unconscious intelligence," and would explain it in a way 

 which differs widely indeed from his. But if such a power 

 is the active agent in such organic adaptations, is it not 

 reasonable to refer to it the special variations which result 

 in the formation of new species ? This is the very activity 

 for the existence of which I have elsewhere contended, 



' Quarterly Journal of Micros. Science, October, 1877, pp. 432, 433, 



