96 THE BEGINNINGS OF LIFE. 



are performed by special parts of the organism, so are 

 the several parts more and more bound together into 

 one organic whole. This difference is well seen in 

 plants and highly-organized animals. The plant, it is 

 true, develops seeds and pollen in special parts or 

 organs; but just as the plant, taken as a whole, is 

 to a great extent a repetition of similar parts whose 

 organization is by no means complex, so do these sepa- 

 rate parts, when severed from the parent organism, 

 retain that reproductive power which enables them, 

 under suitable conditions, to grow into plants of a 

 similar kind. But in animals which are even low in 

 the scale of complexity all this is changed. They are 

 not mere repetitions of separate parts each having 

 a potential individuality of its own : they are rather 

 aggregations of different parts bound together and 

 constituting one organic whole by means of vascular 

 and nervous systems which serve as bonds of unity. 

 The higher the grade of development of the organism 

 the more its tissues have become differentiated the 

 less are they severally endowed with a reproductive 

 power, even of a partial kind. In such organisms, we 

 find that each part has a distinct function to perform, 

 and therefore the reproductive function is restricted 

 to the elements produced in definite organs. Although 

 restricted in their place of origin, however, there is 

 reason to believe that sperm-cells and germ-cells are 

 comparatively unspecialized products they are, indeed, 

 in almost all cases new-formed elements, which have 



