GENERAL BIOLOGY. 43 



simply meant that they came into being in a manner we know 

 not how, in obedience to the will of a First Cause. The other 

 view is denominated the theory of descent with modification, 

 the theory of transmutation, organic evolution, etc., which 

 teaches that all the various forms of life have been derived 

 from one or a few primordial forms in harmony with the recog- 

 nized principles of heredity and variability. The most widely 

 known and most favorably received exposition of this theory is 

 that of Charles Darwin, so that his views will be first presented 

 in the form of a hypothetical case. Assume that one of a group 

 of living forms varies from its fellows in some particular, and 

 mating with another that has similarly varied, leaves progeny 

 inheriting this characteristic of the parents, that tends to be 

 still further increased and rendered permanent by successive 

 pairing with forms possessing this variation in shape, color, or 

 whatever it may be. We may suppose that the variations may 

 be numerous, but are always small at the beginning. Since all 

 animals and plants tend to multiply faster than the means of 

 support, a competition for the means of subsistence arises, in 

 which struggle the fittest, as judged by the circumstances, al- 

 ways is the most successful ; and if one must perish outright, it 

 is the less fit. If any variation arises that is unfavorable in 

 this contest, it will render the possessor a weaker competitor : 

 hence it follows that only usefiil variations are preserved. The 

 struggle for existence is, however, not alone for food, but for 

 anything which may be an advantage to its possessor. One form 

 of the contest is that which results from the rivalry of members 

 of the same sex for the possession, of the females ; and as the 

 female chooses the strongest, most beautiful, most active, or the 

 supreme in some respect, it follows that the best leave the great- 

 est number of progeny. This has been termed sexual selection. 

 In determining what forms shall survive, the presence of 

 other plants or animals is qmte as important as the abundance 

 of food and the physical conditions, often more so. To illustrate 

 this by an example : Certain kinds of clover are fertilized by 

 the visits of the bumble-bee alone ; the numbers of bees exist- 

 ing at any one place depends on the abundance of the field-mice 

 which destroy the nests of these insects ; the numbers of mice 

 will depend on the abundance. of creatures that prey on the 

 mice, as hawks and owls ; these, again, on the creatures that 

 specially destroy them, as foxes, etc. ; and so on, the chain of 

 connections becoming more and more lengthy. 



