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Show 370 PROVISIONAL HYPOTHESIS CIIAP. XXVJf. frequent presence of han. ·s anc1 of pen1e.' c"~1 Y c1 e veloped teeth ' e£v en teeth of the second denti·t i·o n, m· ovaru· m t umour s ' 24 are acts leading to the same conclusion. Whether each of the innumerable autonomous ele.ments of the body is a cell or the modified product of a eel~, IS a mohre doubtful question, even if so wide a d e fi m' tw' n b e given t. ot e . 1 d ll l'ke bodies without walls and without term, as to me u e ce - I . " ermi nal nuclei.25 Professor Lionel Beale uses t_he t~Im. g "matter" for the contents of cells, taken m this WJ~e accepta- tw. n, an d l1 e d raws a br.o a d distinction between germmal matter 26 . l £ d t . 1, anc " orme rna eria 01• the various prod.u cts of cells. But the doctrine of omnis cellula e cellula is admitted ~or pla2~ts, ,and · 'd 1 leut belief with respect to ammals. Thus IS a w1 e y preva . Virchow, the great supporter of the cellular theory, 'vvh1lst allowing tl;at difficulties exist, maintains that eve_ry_ atom of tissue is derived from cells, and these from pre-existmg cells, and these primarily from 1he egg, which he regard~ as a great cell. That cells, still retaining the same nature, mcrease by self-eli vision or proliferation, is admitted by almost every one. But when an organism undergoes a great change of structure during development, the cells, which at each stage are s~ppos_ed to be directly derived from previously-existing cells, must hkewis~ be greatly changed in nature; this change. is apparen~ly attnbuted bv the supporters of the cellular doctrme to some mherent power .,;hich the cells possess, and not to any .external agen~y. Another school maintains that cells and tissues of all kmds may be formed, independently of pre-existin? cells, fro~ _plast~c lympll or blastema ; and this it is thought IS ~ell exhibited m the repair of wounds. As I have not espeCially attended t.o histology, it would be presumptuous in me to express an opinion on the two opposed doctrines. But every one appears to admit that the body consists of a multitude of "organic units," 28 24 I sidore Geoffroy St. Hilaire , 'Hist. des Anomalies,' tom. ii. pp. 549, 560, 562; Virchow, idem, p. 484. ~• For the most recent classification of cells, see Ernst Hackel's ' Generelle Morpholog.,' Band ii., 1866, s. 275. 26 ' The Structure aud Growth of Tissues,' 1865, p. 21, &c. 27 Dr. W. Tmner, ' The pre. cnt Aspect of Cellnlar Pathology,' 'Edin· burgh Medical Journal,' April, 1863. 2s This term is used by Dr. E. 1\fontgomery ('On the Formation of so-called Cells in Animal Bodies,' 1867, p. 42), who denies that cells are derived from other cells by a process of growth, hut believes that they originn.te through eertn.in chemical changes. C HAP. XXVJI. OF PANGENESIS. 371 each of which possesses its own proper attribute~, and is to a certain extent independent of all others. Hence it will be convenient to use indifferently the terms cells or organic units or simply units. Variability and Inheritance.-We have seen in the twentysecond chapter that variability is not a principle co-ordinate with life or reproduction, but results from special causes, generally from changed conditions acting during successive generations. Part of the fluctuating variability thus induced is apparently due to the sexual system being easily affected by changed conditions, so that it is often rendered impotent; and when not so seriously affected, it often fails in its proper function of transmitting truly the characters of the parents to the offspring. But variability is not necessarily connected with the sexual system, as we see from the cases of bud-variation ; and although we may not be able to trace the nature of the connexion, it is probable that many deviations of structure which appear in sexual offspring result from changed conditions acting directly on the organisation, independently of the reproductive organs. In some instances we may feel sure of this, when all, or nearly all the individuals which have been similarly exposed are similarly and definitely affected-as in the dwarfed and otherwise changed maize brought from hot countries when cultivated in Germany; in the change of the fleece in sheep within the tropics; to a certain extent in the increased size and early maturity of our highly-improved domesticated animals; in inherited gout from intemperance; and in many other such cases. Now, as such changed conditions do not especially affect the reproductive organs, it seems mysterious on any ordinary view why their product, the new organic being, should be similarly affected. How, again, can we explain to ourselves the inherited effects of the use or disuse of particular organs ? The domesticated duck flies less and walks more than the wild duck, and its limb-bones have become in a corresponding manner diminished and increased in comparison with those of the wild duck. A horse is trained to certain paces, and the colt inherits similar consensual movements. The domesticated rabbit becomes tame from close confinement; the dog intelligent from associating with man ; the retriever is taught to fetch and carry : and these 2 13 2 |
