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Main thesis is the evolution of species is not fixed and immutable. An organism can belong to a different kind of future generations. From an original, there are a number of different types may have. Evolutionary biologists believe that birds are all descendants of the Chinese nation a certain category of reptiles, two cats and dogs are from a common mammalian stock. Darwinian evolution is not the first time. In the beginning of the nineteenth century, France biologist Ted harmful to Lamarck, the species may take on new forms in response to their needs. Lamarck, and the idea is more subtle than they are often inadequate to describe.           Darwin’s Concept of Natural Selection     The main thesis of evolution is that species are not fixed and immutable. One kind of organism can have descendants that belong to a different kind. From one original species, a number of different kinds may be generated. Evolutionary biologists believe that the birds are all descendants of a particular kind of reptile and that both cats and dogs have come from a common mammalian stock.   Darwin was not the first evolutionist. At the beginning of the nineteenth century, the French biologist Jean Baptiste de Lamarck proposed that species may take on new forms in response to their needs. Lamarck's ideas are more subtle and less definite than they are often portrayed as being. However, a hoary example may serve to distinguish his views from Darwin's. Lamarck would explain the giraffe's long neck as follows. Primitive, short-necked giraffes would have been unable to browse on the leaves of tall trees. Driven by its need for food, each individual primitive giraffe stretched its neck. The giraffes of the next generation benefited from this communal stretching. They inherited the characteristics acquired by their industrious parents. In their turn, they too reached upward. The result was a sequence of giraffes with ever longer necks, a sequence that culminates in the modern form.   This style of evolutionary explanation is not Darwin's. According to Darwin, the principal mechanism for evolution is natural selection. Like the plant or animal breeder, nature selects. Pigeon fanciers (whose doings Darwin studied carefully) choose to breed only those birds with the features that particularly interest them. Analogously, nature "chooses" for survival and reproduction those organisms whose characteristics have best equipped them to compete in a struggle for limited resources. In almost any natural population of organisms, more offspring will be produced than are able to survive. The offspring will vary—in particular, they will vary with respect to characteristics that affect their abilities to survive and reproduce. Some organisms will survive longer and reproduce more frequently. If the advantageous characteristics are inheritable, then they will be transferred to descendants. As a result, they will become more prevalent in later generations. Over a large number of generations the common features of the population may be radically changed.   The idea of evolution by natural selection will be clearer if we contrast Lamarck's giraffe with Darwin's. The Darwinian explanation of the evolution of the giraffe would begin with some initial group of short-necked giraffes (the ancestral population). Though all the giraffes in this group had short necks, some happened to have longer necks than others. These more fortunate beasts were able to browse on foliage that their fellows could not reach. With greater opportunities to feed well, they were better able to survive and multiply. So, in the next generation, the frequency of giraffes with longer necks, and hence average neck length, was slightly increased. Once again, the giraffes with longer necks were at an advantage. After many generations, selection for long necks produced the giraffe of today.   Natural selection shapes the characteristics of plant and animals by working on the variation that naturally arises within a group of organisms. Variation is not directed toward advantageous characteristics. The rigors of the environment do not induce variations designed to cope with them. The organisms of a species are individually different, and nature uses the differences to transform the species.   The Origin of Species showed how the simple idea of evolution by natural selection could be used to illuminate a wealth of biological details. Yet some loose ends were left dangling; important questions were left without firm answers. In particular, Darwin had no clear account of the origin and maintenance of variation in natural populations. He assumed that variations would arise and that the capacity for variation in a particular direction would not be diminished by the operation of natural selection; so, for example, as the average neck length of giraffes increases, giraffes with ever longer necks are supposed to appear. Moreover, Darwin's own hazy ideas about inheritance embroiled him in difficulties. He tentatively accepted the theory of "blending inheritance," holding that the characteristics of the progeny result from "mixing" the attributes of the parents. Several of Darwin's early critics pointed out that this theory makes evolution problematical. If an unusual variation arises in population, then whatever advantages it may confer will be diluted when the distinctive individual mates with other, more mundane, organisms. Quite evidently, the theory of the Origin required better answers to questions about variation and inheritance than Darwin was able to supply.         — Philip Kitcher, Abusing Science: The Case Against Creationism                       Indexes/17