184 EEPORT— 1891. 



contained within a minute ovum, though it may be very great, 

 must yet be limited, and Darwin's theory seems to demand 

 an ahnost infinite number. For not only must each individual 

 character of the animal — anatomical, pliysiological, histological, 

 psychological — down to the minutest, be thus represented in 

 the ovum by its gemmule, but so must each character of all the 

 phases, of its development, and many characters of a remote 

 ancestry. Brooks's modification of Darwin's theory of pan- 

 genesis is designed to avoid this difficulty. According to this, 

 the cells of the body are not constantly throwing off gemmules, 

 but only when the part is subjected to unfavourable condi- 

 tions. 



De Vries regards the protoplasm of the cells of an animal 

 or a plant as made up of ultimate elements which he terms 

 2mngencs. These elements he believes to be much larger than 

 the chemical molecules — each being built up, in fact, of many 

 of the latter, and comparable in size rather to the smallest 

 known organisms. Like Darwin, de Vries believes that each 

 of these particles or pangenes is the bearer of only one single 

 hereditable character ; and so in any given organism there must 

 be as many pangenes as there are hereditable characters. 



In order to attempt to prove this, it is necessary to trace the 

 genealogy of the egg-cell from one generation to the next. In 

 some plants, as is well known, such as the OscillaricB, there is 

 only a single kind of cell, evei-y cell being capable of acting as 

 a reproductive cell. In most plants, however, and in all the 

 higher forms, there are a variety of sets of cells, some of which 

 — the purely somatic cells — are incapable of giving rise to the 

 reproductive cells, while others are \\\ the direct line from the 

 reproductive cells of the parent plant to those about to be 

 developed. The series formed by the latter lead from egg-cell 

 to egg-cell (or sperm-cell to sperm-cell) either directly — gene- 

 rally within one individual — or indirectly by a roundabout way 

 by vegetative reproduction, the series often passing through a 

 considerable number of individuals before coming back again 

 to the egg- cell. Such a cell-genealogy is unknown in the 

 higher types of the animal kingdom, where the descent from 

 one generation of egg-cells to the next is short and direct. It 

 is very general among plants, however, more particularly 

 among the Thallophyta and Mviscineae, where in some in- 

 stances every fragment of the plant is capable, when detached, 

 of giving origin to a new individual. 



In the higher plants there are found present in increasing 

 numbers genealogical lines of cells which are essentially somatic 

 — i.e., which are destined to give rise to non-productive ele- 

 ments ; and in vascular plants there can be no doubt that, at 

 least in the mature condition, tissue-cells can no longer repro- 

 duce the species. 



