PRESIDENTIAL ADDRESS. 463 



1 may point out in conclusion that general discussions of this kind may be 

 useful in other ways than as attempts to discover truth or as a striving towards 

 a verity which is indefinable and perhaps unattainable. Even if my scheme 

 of evolution be but a midsummer-night's fantasy, I claim for it that it co- 

 ordinates a number of isolated and scattered phenomena into an orderly, and, 

 I think, intelligible sequence, and exhibits them in a relationship which at 

 least enables the mind to obtain a perspective and comprehensive view of them. 

 Rival theories will be more, or less, useful than mine, according as they succeed 

 in correlating more, or fewer, of the accumulated data of experience. If in 

 this address I succeed in arousing interest and reflection, and in stimulating 

 inquiry and controversy, it will have fulfilled its purpose. 



Appendix A. — The Cell-Theory. 



The most recent attack on the Cell-theory, as it is understood by the 

 majority of modern biologists, has been made by Mr. Dobell, who, if I under- 

 stand him rightly, refuses to admit any homology between the individual 

 Protistan organism and a single cell of the many that build up the body of a 

 Metazoon. Oni the contrary, he insists that the Protist is to be regarded as 

 homologous with the Metazoan individual as a whole. On these grounds he 

 objects to Protista being termed ' unicellular ' and insists that the term ' non- 

 cellular ' should be applied to them. 



As regards the cellular nature of the Protista, it is one of my aims in this 

 address to show that amongst the Protista all stages of the evolution of the cell 

 are to be found, from primitive forms in which the body cannot be termed a 

 cell without depriving the term ' cell ' of all definable meaning, up to forms of 

 complex structure in which all the characteristic features of a true cell are 

 fully developed. Thus in the Protozoa we find the protoplasmic body 

 differentiated into nucleus and cytoplasm ; the nucleus in many cases with a 

 structure comparable in every detail to that of the nucleus of an ordinary body- 

 cell in the Metazoa; reproduction taking place by division of the body after 

 a karyokinetic nuclear division often quite as complicated as that seen in 

 the cells of the Metazoa and entirely similar both in method and in detail ; 

 and in the sexual process a differentiation of the gametes on lines precisely 

 similar to those universal in Metazoa, often just as pronounced, and preceded 

 also in a great many cases by phenomena of chromatin-reduction comparable in 

 principle, and even sometimes in detail, with the reduction-proces.^e.'; occurring 

 in Metazoa. I really feel at a loss to conceive what further criteria of 

 homology between a Protozoon and a IMetazoan cell could be demanded by 

 even the most captious critic. On the ground of these and many other simi- 

 larities in structure and behaviour between the entire organism in the Protozoa 

 and the individual cell, whether tissue-cell or germ-cell, in the Metazoa, the 

 case seems to me overwhelmingly convincing for regarding them as truly — 

 that is to say, genetically — homologous. 



Looking at the matter from another point of view, namely, from the stand- 

 point of the Metazoa, it is true that in the groups of most complicated and highly 

 organised structure the cells often develop secondary connections or fusions due 

 to incomplete division, to such an extent that in parts of the body the individu- 

 ality of the primitively distinct cells may be indicated only by" the nuclei (a.s 

 may occur also in Protozoa, for example, in associated gregarines) ; but in all 

 Metazoa certain of the cells retain permanently their complete independence 

 and freedom of movement and action. In the Metazoa possessing the simplest 

 and most primitive types or organisation, such as sponges and coelenterates, the 

 cells composing the body show far greater independence of action, and in the 

 course of ontogeny entire groups of cells may alter their relative positions in 

 the body as the result of migrations performed by individual cells; while it is 

 now well known that if the adult sponge or hydroid be broken up completely 

 into its constituent cells, those cells can come together again and build up, 

 by their own individual activity, the regenerated body of the organism. For 

 the.se reasons it seems to me impossible to regard the body-cells of the Metazoa 

 otherwise than as individual organisms complete in themselves, primitively as 

 independent as the individual Protozoon, and in every way comparable to it. 



From the considerations summarised very briefly in the two foregoing para- 



