310 ANNUAL OF SCIENTIFIC DISCOVERY. 



tion, and the conditions which produce an inversion of the viscera 

 established at the will of the experimenter. He has shown that 

 the incurvation of the cardiac fold to the left of the embryo, the 

 cause of visceral inversion, may be produced simply by so placing 

 the eggs that their long axis shall be oblique to the axis of the 

 heating tubes, and that the pointed should be higher than the ob- 

 tuse end. To render the inversion certain, there must be also a 

 certain depression of the surrounding temperature, at the points 

 not artificially heated ; the inversion did not occur unless the sur- 

 rounding air was below 22° C. 



Darwin's theory of pangenesis. 



While laying great stress on the conditions of existence as the 

 great cause of the modification of species, and as one of the main 

 causes of change in the organs of reproduction, and also on 

 "natural selection" as a modifier of the egg, the seed, the young, 

 as well as the adult, Darwin finds in what he calls his provisional 

 h3'pothesis of "pangenesis" the solution of the problem and the 

 immediate means by which the change is effected. In his own 

 words, this hypothesis "implies that the whole organization in the 

 sense of every atom or unit reproduces itself. Hence ovules and 

 pollen-grains — the fertilized seed or egg, as well as buds — in- 

 clude and consist of a multitude of germs thrown off from each 

 separate atom of the organism." It is, in the words of a writer 

 in the " Quarterly Journal of Science" for July, 1868, the applica- 

 tion of the atomic theory to living forms, and is in perfect con- 

 formity with all the teachings of correlation between vital and 

 physical forces." — "The author believes that all changes in the 

 various organisms which result from the contact of the spermato- 

 zoon and ovum, as well as those which are derived from gemma- 

 tion or budding, have the'^* origin in the nature of the cells which 

 constitute the elements or materials in operation. The cells or 

 units which constitute all living bodies, from the simplest to the 

 most complex, are themselves organized, and consist of lesser 

 cells or atoms having various natures, and, according to the 

 author, they give off those constituent atoms as ' gemmules,' 

 and the nature of those atoms or gemmules fixes the future char- 

 acter of the organism into which they enter. In this manner he 

 seeks to account for the first variation in living types ; for the 

 transmission of inherited peculiarities from a grandfather, say, 

 through a daughter to a grandchild ; for hybridism." 



Darwin says, at the conclusion of his chapter on pangenesis : 

 " Finally, the jDOwer of propagation possessed by each separate 

 cell, using the term in its largest sense, determines the reproduc- 

 tion, the variability, the development, and renovation of each 

 living organism. Each living creature must be looked at as a mi- 

 crocosm, — a little universe formed of a host of self-propagating 

 organisms, inconceivably minute, and as numerous as the stars in 

 heaven." 



For a further development of the theory the reader is referred 



