& Q Natural Selection. 

 Origin of Species 



IT IS certain that the plants and animals of 

 today have descended from species that oc- 

 cupied the earth in previous times. Moreover, 

 there are only two alternatives as to the na- 

 ture of this descent. Either there has been a 

 fixity of the species, such that the existing 

 species have descended without change from 

 the pre-existing species; or evolution has oc- 

 curred, in the sense that the species have 

 undergone change during the course of their 

 descent from pre-existing forms. 



The idea of evolution extends back into 

 ancient times, but Darwin, in 1859, first suc- 

 ceeded in marshaling a convincing array of 

 basic evidence. However, it was not until 

 the present century that modern genetics 

 provided many missing clues as to how the 

 known processes of heredity and reproduc- 

 tion inevitably lead to evolution. 



Under natural conditions every species of 

 plant and animal is constantly subjected to 

 a process of selection. The parents selected 

 to perpetuate each species in every genera- 

 tion are not chosen artificially according to 



the specifications of mankind, but naturally, 

 according to their ability to live and repro- 

 duce under the conditions of their particular 

 environment. This process of natural selec- 

 tion, as will be apparent shortly, results in- 

 evitably from the normal tendency of every 

 organism to multiply at a rate that exceeds 

 the capacity of the environment to support 

 all the offspring of any given species. 



MULTIPLICATION OF ORGANISMS; A 

 GEOMETRICAL PROGRESSION 



Every kind of organism tends to multiply 

 at a characteristic rate. Perhaps the highest 

 rate occurs among certain bacteria, which, 

 under favorable conditions, will divide once 

 every half hour. At this rate the population 

 would be quadrupled every hour, and in 24 

 hours the descendants of a single bacterium 

 would number 2 48 (more than 280 million 

 million), // they all survived, A million 

 seeds, or spores, per year are not uncommon 

 for an individual plant, and many inverte- 



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