THE CONCEPT OF EVOLUTION J\$ 



the intermediate compounds. W^hen, by other series of mutations, the 

 organism was able to synthesize all of its requirements from simple 

 inorganic compounds, as the green plants can, it would have become 

 an autotroph. Once the first simple autotrophs had evolved, the way 

 was clear for the further evolution of the vast variety of green plants, 

 bacteria, molds, and animals that inhabit the world today. 



These considerations lead us to the conclusion that the origin of 

 life, as an orderly natural event on this planet, was not only possible, 

 it was almost inevitable. Furthermore, with the vast number of planets 

 in all the known galaxies of the universe, many of them must have 

 conditions which permit the origin of life. It is probable, then, that 

 there are many other planets on which life as we know it exists. 

 W^herever life is possible, it should, if given enough time, appear and 

 ramify into a wide variety of types. Some of these may be quite dis- 

 similar from the ones on this planet, but others may be quite like 

 those found here; some may, perhaps, be like ourselves. 



It seems unlikely that we will ever know how life originated, 

 whether it happened only once or many times, or whether it might 

 happen again. The theory (1) that organic substances were formed from 

 inorganic substances by the action of physical factors in the environ- 

 ment; (2) that they interacted to form more and more complex sub- 

 stances, finally enzymes, and then self-reproducing enzyme systems 

 ("free genes"); (3) that these "free genes" diversified and united to form 

 a primitive, j^erhaps virus-like heterotroph; and (4) that autotrophs then 

 evolved from these heterotrophs, has the virtue of being quite plausible. 

 Many of the parts of this theory have been subjected to experimental 

 verification and have been shown to be feasible. 



306. Principles of Evolution 



However much students of evolution may disagree as to the nature 

 of mutations, the kind of mutations involved in evolution, and the de- 

 gree to which such factors as natural selection, isolation, genetic re- 

 combination and population dynamics may affect the evolution of some 

 particular organism, there are several fundamental principles upon 

 which they are agreed: changes within the genes and chromosomes are 

 the raw material of evolution, some sort of isolation is necessary for 

 the establishment of a new species, and natural selection is involved in 

 the survival of some, but not all, of the mutations which occur. In addi- 

 tion, there are five principles of evolution to which nearly all biologists 



would subscribe. 



1. Evolution occurs more rapidly at some times than at others. At 

 the present time it is occurring rapidly, with many new forms appearing 

 and many old ones becoming extinct. 



2 Evolution does not proceed at the same rate among different types 

 of organisms. At one extreme are the lampshells or brachiopods, some 

 species of which have been exactly the same for the last 500,000,000 

 years at least, for fossil shells found in rocks deposited at that time are 

 identical with those of animals living today. In contrast, several species 



