EVOLUTION, HOMOPLASY, ANALOGY, HOMOLOGY 339 



origin of the various forms of Plant-Life was by gradual Evolution 

 through long ages of Descent, it must be clearly understood that this 

 is nothing more than a theory. Evolutionary History as a whole is 

 not, and cannot be, a result of actual demonstration. It has not 

 been possible to produce detailed proof of the evolutionary history of 

 any wild species, or genus, whatever the probability may seem to be 

 as to the source from which it sprang. But so many facts can be 

 explained by the theory of Evolution that it is now generally accepted 

 as giving the most intelligible account of the probable origin of the 

 Organic World as we see it. 



The evidence upon which its probability rests, is derived (i.) from 

 Comparisons based on external form and internal structure : (ii.) from 

 a study of the Ontogeny, which means the history of development of 

 the individual from the egg to the adult : (iii.) from Palaeontology, 

 which means the study of fossils in their stratigraphical sequence : this 

 demonstrates their Distribution in Time : and (iv.) from the evidence 

 given by the Geographical Distribution of organisms upon the Earth's 

 surface at the present day. If the theory be true, the results derived 

 from these several methods of enquiry should coincide, and that 

 coincidence should apply not only for Plants, but for the Animal 

 Kingdom as well. It is the high degree of coincidence seen in the 

 evidence derived from all of these sources, for the two Kingdoms 

 of Living Beings, which has led to the general acceptance of a Theory 

 of Evolution. 



The facts of Variation and of Heredity constitute the basis upon 

 which Evolution is believed to have worked. In Plants, as in 

 Animals, the offspring shows a general resemblance to the parents. 

 But that resemblance, however close, is not exact. The individual 

 variations from type may be greater or less : often they are very 

 slight indeed. They differ also in respect of their hereditary trans- 

 mission to a succeeding generation. Some are not transmitted, so 

 far as present evidence goes, and consequently they are not available 

 in Evolution. These are called Fluctuating Variations or Modifications. 

 Others are evidently heritable and are transmitted to the offspring ; 

 these are called Mutations. The problem of variation is dealt 

 with more fully in Chapter XXXV. In the meantime it may be 

 noted that any process in Nature that would bring about a 

 summing together or accumulation of the heritable differences which 

 exist would tend to establish in the race such differences as characterise 

 species, genera, and successively larger groups. Such a process is 

 found to be actually at work in Natural Selection. It produces its effect 



