10 EVOLUTION OF LIVING ORGANISMS. 



the non-living can never have been bridged. On the 

 contrary, although far from being able to give a complete 

 scientific explanation of all the phenomena of life, we 

 have made so much progress towards that final goal of 

 the evolutionist that we seem fully justified in believing 

 that the transition from the non-living to the living has 

 indeed occurred, and even in hoping that some day the 

 very origin of life will be explained. 



The organisms now living on this earth are known as 

 plants and animals. The study of plants is called 

 Botany, that of animals Zoology ; while Biology is the 

 name given in England to the science of living things in 

 general, including both animals and plants. Now one of 

 the first results of a deeper knowledge of living organ- 

 isms is to show us how much there is in common between 

 them, and how much they differ from their non-living 

 surroundings. There can be no doubt that even the 

 simplest plant or animal now living is the product of a 

 long series of evolutionary changes the initial steps of 

 which have long since disappeared, or are at all events 

 unknown to us. 



In the first place, all living matter contains substances 

 of peculiar molecular structure and composition, far more 

 complex than any compounds found in inorganic nature. 

 Such complex compounds occur only in living matter 

 itself, or in its products. But the difference between 

 these organic and the inorganic substances is only one of 

 degree, and many of the most characteristic of them have 

 been artificially made in the chemical laboratory. 



Built up of the ordinary elementary chemical sub- 

 stances of nature, chosen indeed as we might expect from 

 among the commonest elements on the surface of the 

 earth, these organic compounds may be grouped in three 

 classes : carbohydrates (such as sugars, starch, and 

 cellulose), fats, and proteins. Of these the proteins are 

 by far the most important. For while the molecule of 

 fat or carbohydrate consists entirely of various combiua- 



