72 THE THEORY OF NATURAL SELECTION 
first sight to be no inherent difficulty in the way of 
acquired characters being inherited. Weismann has, 
however, pointed out a very serious difficulty, which 
is brought into prominence on making a study of the 
minute anatomy of the cells of organisms during the 
earlier stages of their development. 
In the ordinary course of events every one of the 
higher animals and plants begins its existence in the 
form of a single minute cell—the fertilized ovum or 
egg. This cell exhibits no trace of the complicated 
series of organs which will develop from it when it is 
subjected to the proper conditions. When the egg 
is placed in favourable circumstances with regard 
to warmth, moisture, food-supply, and the like, it 
first divides into two equal portions; and microscopic 
study shows that elaborate precautions are taken to 
insure the equal bipartition of its minute constituent 
parts. Each of the two cells thus formed divides 
again into two further cells, and by a series of repeated 
bipartitions of this kind the cells which constitute the 
adult body are at last brought into existence. Since 
the body soon becomes differentiated into a number of 
unlike organs, it is clear that at certain stages of the 
process the two cells arising from a division must come 
to differ slightly from one another; and the cells ulti- 
mately produced show very considerable differences 
of form, structure, and size. Among all the cells which 
finally arise those which have undergone the least 
modification from their original condition are those 
from which are developed the sexual reproductive cells, 
or germ-cells, of the organism. Indeed, Weismann con- 
