50 COMPARATIVE PHYSIOLOGY. 



and moral nature of man. Wallace believes that man's body 

 has been derived from lower forms, but that his higher nature 

 is the result of some unknown law of accelerated development ; 

 while Darwin, and those of his way of thinking, consider that 

 mau in his entire nature is but a grand development of powers 

 existing in minor degree in the animals below him in the scale. 

 Summary. — Every group of animals and plants tends to in- 

 crease in numbers in a geometrical progression, and must, if 

 unchecked, overrun the earth. Every variety of animals and 

 plants imparts to its oflPspring a general resemblance to itself, 

 but with minute variations from the original. The variations 

 of offsprings may be in any direction, and by accumulation 

 constitute fixed differences by which a new group is marked 

 off. In the determination of the variations that persist, the law 

 of survival of the fittest operates. 



