32 ORGANIC EVOLUTION— PHYSICAL 



we may conveniently begin the study of life at that 

 stage of evolution which they have reached. Watched 

 under the microscope the amoeba is known by its 

 actions to be a living being. It puts forth or with- 

 draws thinner or thicker processes known as pseudo- 

 podia. It moves by flowing forward, or by putting 

 forth pseudo-podia and flowing into them. Streaming 

 motions of granules may be observed in its substance. 

 It engulfs food particles, and having assimilated the 

 digestible portions, flows away, leaving the indigestible 

 remainder behind. It shrinks from harmful contact. 



In describing the amoeba I have described also the 

 white blood corpuscle, the leucocyte of man and other 

 animals. These occur in great numbers, maintaining a 

 separate existence in the blood, or wandering through 

 the tissues. It might be thought that they are para- 

 sites, but this is disproved by the fact that if any tissue 

 is injured, as for instance by a cut, they crowd in count- 

 less numbers to the spot, and repair the injury with 

 their bodies, which thereupon undergo changes in shape 

 and structure. What is known as pus or " matter," such 

 as flows from an abscess, is a clear fluid rendered turbid 

 by the multitude of their dead. In cases of zymotic 

 disease they have been seen with the invading microbes 

 enclosed in their substance, when either the leucocyte 

 or the microbe perishes. 



Both the amoeba and tlie leucocyte multiply by 

 fission, by dividing the one into two. First the 

 nucleus divides by a complicated process known 

 as Karyokinesis, the completion of which is followed 

 by the division of the cell body. The daughter-cells 

 grow, and in time divide in the same manner as the 

 parent. This process is repeated through an immense 

 tliough finite number of generations by the leucocyte, 

 and through an infinite number of generations by the 

 amoeba. Whence it is clear that the amoeba is 



