litter can be very different, as well as seedlings from the same fruit. He also goes on to explain the differences in pigeons. Of all the different breeds, there are extreme variations in beak size, skull shape, length of wings, and facial features such as eyelids. Another example was shown to Darwin by Mr. Lubbock. Darwin explains; "I should never have expected that the branching of the main nerves close to the great central ganglion of an insect would have been variable in the same species; I should have expected that changes of this nature could have been effected only by slow degrees: yet quite recently Mr. Lubbock has shown a degree of variability in these main nerves in Coccus..."Now that Darwin was sure that variations occur within species, he tried to explain why. He was exactly right in his reasoning, even despite the fact that he had no concept of the gene. Darwin thought that there are a number of causes, but in general the cause is the environment. Pertaining to cultivated plants, different climate, treatment, and excess food all will cause a variation from the parent that grew in nature. The conditions in which the parent grew were far different than that in which the cultivated plants would grow, and this, Darwin explains, is the cause of variation. However, he goes on to give several other possibilities that might cause variation. Reproduction is mentioned, for example the treatment of the ovules or pollen. In general, he believed that the conditions of life were responsible for variability.The Struggle for Existence, Natural Selection, Extinction The struggle for existence, according to Darwin, is due to the geometrical growth in which plants and animals increase. If more animals are produced than can survive (due to shortage of food, space, etc.), then there will be competition between and within species. Darwin uses man as an example. He states, "Even slow-breeding man has doubled in twenty-five years,...
