人教全日制普通高中英语（必修）高三UNIT4-2

 In his youth Charles Darwin enjoyed all the freedom to experiment and do whatever he liked. The study of physics, chemistry, and later botany, was his hobby. At the age of 22, Darwin was invited to join the scientific expedition on the Beagle. During its five-year voyage, the Beagle followed almost the same route as Cook's Endeavour sixty years earlier. Darwin's observations on that voyage led him to write his famous book On the Origin of Species 1.

 

        It was Darwin's visit on the Beagle to the Galapagos Isles 2 that gave him the key to his new theory. Darwin soon realised that the wildlife of the Galapagos Isles was very curious and deserved 3 special attention. Most of the life forms could be found nowhere else. There were even differences between the species of the different islands; yet all showed a clear. relationship with those of America, though separated from that continent by about 600 miles. Darwin counted 225 flowering plants, of which he collected and brought home 193. Of these, 100 were new species which are only found in the Galapagos islands. Altogether the group of islands formed a little world within itself.

 

        Darwin first understood the development of different plant species from developments in bird species that he observed in Galapagos. He found 13 kinds of finches, related 4 to each other in the structure of their beaks 5, tails, shape of body and colour of their feathers. The most curious fact was the differences in the size of the beaks. This could only be explained by the differences in habitat and source of food. The birds with broad beaks would eat hard seeds, while the birds with smaller beaks would eat soft seeds or insects. Back home, in England, Darwin realised that differences in habitat could lead to different species in birds as well as in plants. He had found the key to explaining differences between species.

 

       During the 1840s and 1850s Gregor Mendel conducted his experiments with flowers and peas. Mendel's attraction to research was based on his love of nature. Although his experiments were designed to support the ideas concerning 6 the influence of environment upon plants, he found that many characteristics were passed on from one generation to the next, without influence by the environment. His research gave birth to the science of genetics.

 

       As a result of Darwin and Mendel's research, scientists of the nineteenth century formed the belief that the influence of the environment was behind the development of new species. Still, both Darwin and Mendel observed that within species, different varieties could occur. Darwin studied many varieties of garden roses. It would take a next generation of scientists to bring the importance of the environment on species back in view.

 

       In 1922, G6te Turesson found evidence for the existence of stable varieties within species in nature. He showed that differences between plants of one species occurred as a result of the environmental conditions in their habitat. He made his discoveries by studying a wild plant found on the Swedish west coast. When growing on rocks, the plant grows to a bush, having broad leaves and large flowers. However, when growing on sand, the plant grows close to the ground, having narrow leaves and small flowers. If plants from one habitat were moved to the other type. of habitat, they changed their appearance and adapted to the new environment.

 

       The research by Darwin, Mendel and Turesson shows that both genetics and the environment are important to plants. For the improvement 7 of crops Mendel's genetics were long considered the most important. However, Turesson's findings 8 show that habitat, such as soil conditions, is of equal importance to the output 9 of crops.