260 Master Minds of Modern Science 



by degrees the science of biology developed until biologists 

 penetrated beneath the surface and began to study the 

 organs of animals and the tissues of plants. Schleiden 

 the botanist discovered in 1838 that all plant substances 

 were built up of cells. To-day natural history goes so 

 deep that it has become closely allied to chemistry and 

 physics. 



One of the merits of biology is that it teaches man so 

 much about his own beginnings. Man has been on the 

 earth for a very long time, yet, comparatively speaking, 

 he is a newcomer. Says Sir Arthur : 



If we could arrange a great cinema film of the evolution of 

 living creatures, giving proportionate lengths to the successive 

 organic dynasties, arranging the whole so that it could be un- 

 rolled at uniform rate throughout a day, beginning at nine in 

 the morning, then man would appear a few minutes before 

 midnight. . . . Yet man only, among all living creatures, is 

 aware of the long drama, and even he has but a dim under- 

 standing of the plot. 



It is through the work of men such as Sir Arthur 

 Thomson that we begin to have some idea of the begin- 

 nings of life on this planet. At first the earth was a ball 

 of flaming vapour, which gradually cooled and contracted 

 until it had a solid crust. A most unpleasant crust, for it 

 was smoking and cindery, and the atmosphere, such as it 

 was, would have poisoned any living being. There was 

 certainly very little oxygen, for most of the oxygen in 

 the air has been made out of carbonic acid gas by green 

 plants working in sunlight. At first there was no sun- 

 light at all. The light was cut off by enormous masses of 

 cloud, such as still surround the planet Venus. 



By degrees the earth's crust cooled, rain began to fall, 

 and pools of water appeared. In the course of ages these 

 pools grew to seas, which dissolved the salts out of the 

 earth and themselves became salt. It is possible that at 



