THE CELL THEORY 107 



plants, which are not always to be clearly separated) are 

 now known to be autonomous cells, increasing by fission, 

 and often forming- colonies. Conjugation (fusion of 

 similar individuals) often precedes fission, and when it 

 was proved (1861-5) that ova and spermatozoa are true 

 cells, it was seen that fertilisation, as we know it in the 

 higher animals, is only a special form of the conjugation 

 observed among the Protozoa. To the Protozoa it is 

 now possible to trace, without any startling break of 

 continuity, all the multicellular organisms, their tissues, 

 the growth of those tissues by repeated fission, their 

 eggs, and the process of fertilisation which precedes 

 cleavage. The old Greek riddle, '* Which came first, 

 the fowl or the egg ? " may now receive the answer : 

 " Neither ; their common starting-point is to be found 

 in the Protozoa, which, even when adult, represent the 

 primitive unicellular condition, to which all the higher 

 animals revert once in every generation." 



It is not without reason that biologists dwell on the 

 unifying influence of the cell-theory, which has become 

 a chief support of that still wider unifying influence, the 

 Origin of Species by Natural Selection. When it was 

 discovered that all living things, whether plants or 

 animals, consist of nucleated cells which increase by 

 fission, and that in all of them cell-fission is started 

 anew from time to time by a cell-fusion, it was strongly 

 suggested that resemblances so striking and so universal 

 can only proceed from a common descent. 



During the last half-century the study of cells has led 

 to a great increase of knowledge respecting all bodily 

 functions, whether in health or disease. We now look 

 to it as perhaps the most hopeful source of new light 

 upon the important question of hereditary transmission. 



H 2 



