316 THE EVOLUTION THEORY 



was so among animals, it could not have been dispensed with among 

 plants either. 



Moreover, throughout the vegetable kingdom divisions often 

 occur in connexion with the origin of the sex-cells which can be 

 compared, in occurrence and result, with the maturation divisions of 

 animal germ-cells. In the lichen, Baaidioholus, we have already seen 

 that an abortive cell separates itself off from the sex-cell before the 

 latter becomes capable of reproduction (Fig. 8i, C). Similar cell- 

 divisions occur in many if not in all groups of plants. In the marine 

 algaB of the genus Fucus it has even been proved that the division of 

 the first primordial cell of the ovary into the so-called ' stalk-cell ' 

 and the primitive egg-cell is a reducing division, and brings down the 

 number of chromosoines from thirty-two to sixteen. In vascular 

 plants the reduction is not postponed until the formation of the sex- 

 cells, but occurs earlier in the formation of the spores, as Calkins has 

 demonstrated for ferns; in the Conifers and other Gymnosperms 

 several so-called ' preparatory ' divisions precede the formation of the 

 germ-cells, and we know by comparison with the alternation of 

 generations in vascular plants that these are related to the gradual 

 waning of the strictly sexual generation. As the ' polar bodies ' or 

 '. directive corpuscles ' of the animal ovum are rudimentary egg-cellS) 

 so the cells which, in the pollen-grains, separate themselves from the 

 sex-cells proper are rudimentary Prothalliuni-cells, and, like the 

 animal cells, they perish without playing any further physiological 

 role. I will not assert that it is precisely in these divisions that the 

 reducing divisions are concealed, for the analogy with the spore- 

 formation of ferns leads us rather to suppose that it may lie further 

 back ; but in any case there is no lack of opportunity in the ontogeny of 

 phanerogamic plants for the interpolation of a reducing division, and 

 as long as it remains unproved that a rediiction of the chromosomes 

 can take place directly, that is, without the help of nuclear division, 

 we shall continue to expect with confidence that the reducing divisions 

 of phanerogams will be discovered in the future. Processes of a 

 similar kind are known among unicellular organisms, and there, too, 

 they are associated with nuclear divisions. 



In passing to the so-called 'sexual reproduction' of unicellular 

 organisms, I should like first to call attention to the fact that the 

 expression 'reproduction' is not very suitable in this case, for the 

 process in question does not always effect an increase in the number 

 of individuals as reproduction ought to do, but leads, in fact, in many 

 cases, even to a decrease, when two individuals unite to form one. 

 Even if the phenomena of sexual 'reproduction' amoncr higher 



