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- 



