52 THEORIES OF HEREDITY AND VARIATION. 



The third law and its explanation is now known as the "Law 

 of Use and Disuse," or kinetogenesis ; the fourth law, as "Inherit- 

 ance of Acquired Characters;" and the two of them together as 

 "Lamarckian Factors." 



We have in Lamarck's laws a clear and distinct statement of 

 the cause of variations, but Lamarck did not give any adequate 

 proof of their truth. Neither has any one since Lamarck's time 

 been able to give a proof that was entirely satisfactory, though many 

 naturalists believe that these laws are a true statement of facts. 



WEISMANN'S THEORY. 



The opposing ideas are best represented by the theory of August 

 Weismann, a German embryologist who has carried his investiga- 

 tions back to the earliest known source of life. The most primitive 

 forms of animal life consist of minute rounded bodies of gelatinous 

 substance. These bodies are called "cells," and each is a complete 

 individual in itself. An individual which consists of a single cell 

 is called unicellular, and unicellular organisms are generally desig- 

 nated by the term plasm or protoplasm. Individuals which consist 

 of a number of cells grouped together are called multicellular, and 

 multicellular organisms are a step higher in the scale of nature. 

 In unicellular organisms the cell grows for a time, then there appears 

 around it an equatorial depression like a string tied around the 

 center of a pillow. This depression gets deeper and deeper until 

 the two halves are finally separated and float away as two complete 

 cells. These new cells again grow and each again divides in the 

 same manner. In the lower forms of multicellular organisms an 

 individual consists of a certain number of cells, say sixteen. These 

 cells grow to a certain size, when each cell will divide into two 

 cells of smaller size, making an individual of thirty-two cells. The 



