DEFINITION OF SOME MODERN SCIENCES. 117 



past the great cities, illustrates how clearly everybody sees natural 

 law at work in society. It is the laws of society that determine 

 the direction in which population moves. For example, in peoples at 

 all advanced the head of navigation of rivers is usually the site of the 

 principal towns. A short time ago, when water was more used than 

 now as a power, there was usually combined with the advantages offered 

 by the head of navigation (vessels being then smaller than now), the 

 additional advantage of the fall in the stream, which is almost always 

 greatest at the point where the piedmont plateau joins the coastal 

 plain. As streams only reach their base level after emerging upon the 

 coastal plain, this sudden fall almost always occurs a short distance 

 above the head of navigation. As this is true of all the streams that 

 drain a continent, a line may be drawn through this point on all the 

 rivers and it will be approximately parallel to the coast. Such a line is 

 called the 'fall line,' and it is a law of population that the first settle- 

 ments of any country take place along the fall line of its rivers. 



There are many laws that can be similarly illustrated, and careful 

 observation reveals the fact that all social phenomena are the results of 

 its laws. The one example given must suffice in the present case. But 

 these social, or sociological, laws may themselves be grouped and 

 generalized, and higher laws discovered. If we carry the process far 

 enough we arrive at last at the fundamental law of everything psychic, 

 especially of everything affected by intelligence. This is the law of 

 parsimony. It has, as we shall see, its applications in biology, and its 

 homologue in cosmology, but it was first clearly grasped by the political 

 economists, and by many it is regarded as only an economic law. Here 

 it is usually called the law of greatest gain for least effort, and is the 

 basis of scientific economics. But it is much broader than this, and not 

 only plays an important role in psychology, but becomes, in that collec- 

 tive psychology which constitutes so nearly the whole of sociology, the 

 scientific corner stone of that science also. 



We have seen that the quality of scientific exactness in sociology can 

 only be clearly perceived in some of its higher generalizations, where, 

 neglecting the smaller unities which make its phenomena so exceedingly 

 complex, and dealing only with the large composite unities that the 

 minor ones combine to create, we are able to handle the subject, as it 

 were, in bulk. Here we can plainly see the relations and can be sure 

 of their absolute uniformity and reliability. When we reach the law 

 of parsimony we seem to have attained the maximum stage of general- 

 ization, and here we have a law as exact as any in physics or astronomy. 

 It is, for example, perfectly safe to assume that under any and all 

 conceivable circumstances, a sentient and rational being will always 

 seek the greatest gain, or the maximum resultant of gain — his 

 'marginal' advantage. 



