388 Weismann s Theories — 



totle (also expressed by Empedocles) have become solidly 

 established through the researches of modem science. We 

 now know that both these influences play their part, although 

 it is still keenly debated whether or not there is any essential 

 difference between them. This question, with the problem 

 how and why the offspring resembles the parent to the 

 wonderful extent it may, are amongst the subjects treated of 

 in Professor Weismann's essays. Another relates to the first 

 introduction of death into the world, and its bearing upon 

 that 'struggle for existence' between species and species, 

 with which the writings of Darwin and of Wallace have made 

 us all familiar. He considers the whole mass of living 

 animals and plants as divisible into two great groups, be- 

 tween which the strongest contrast exists both as to structure 

 and vitality. The overwhelming majority of animal and 

 vegetable organisms consists, as we do ourselves, of different 

 kinds of substances — different tissues ^ — each made up of, or 

 having been formed from, a multitude of cells.^ A natural 

 death is now the inevitable fate of all creatures thus com- 

 plexly formed. But there is another set of animals and 

 plants, mostly of minute size, which each consist of but a 

 single cell, and none of these, according to Professor Weis- 



1 Such, e.rjr., as muscle, nerve, bone, blood, etc., each of which is a different 

 kind of 'tissue.' 



2 The minute structures termed * cells' were first observed in plants, 

 which indeed are mainly composed of them. Each cell is a minute bag with 

 fluid contents, in which there is commonly present a certain denser body 

 termed the * nucleus,' as was shown by the illustrious botanists Robert Brown 

 and Schleiden. This generalisation was subsequently extended to the animal 

 kingdom by Schwann, and thus that general conception known as 'the cell- 

 theory ' was promulgated. It became generally accepted about 1840. I)i 

 1849 Nageli showed that the wall of the bag might be wanting, and by 1854 

 Max Schultze and others had shown that the nucleus also might be absent. 

 Thus the ' cell ' came to be regarded as a minute portion of semifluid sub- 

 stance, to which the now famous term of ' Protoplasm ' came ultimately to 

 be applied. This term was first used in a definite manner by Van Mohl, 

 who employed it to denote the soft contents of the cell of plants. Within 

 the nucleus of a cell a still smaller body is to be found known as the 

 nucleolus, or nucleoli if more than one. Recent observers have discovered 



