DIV. 



MORPHOLOGY 209 



reduced stamens or the appearance of reduced or fertile stamens in 

 positions where fertile stamens were present in the ancestry. The 

 similarity of the embryos of very different organisms, which is most 

 strikingly shown in the animal kingdom, is a further indication of 

 genetic relationship. So also is the fact that occasionally the embryos 

 are more highly organised than the mature organism (in some reduced 

 organisms, e.g. many parasites). The juvenile leaves on the seedlings 

 of some plants which are adapted to extreme conditions of life may 

 resemble the ordinary leaves of less specialised species of the same 

 genus (e.g. in Acacia, Fig. 136). Not infrequently a species repeats 

 more or less completely in its ontogenetic development what we assume 

 on other grounds to have been the course of its phylogenetic 

 development (BIO<?ENETIC LAW). 



3. EVIDENCE FROM GEOGRAPHICAL DISTRIBUTION. Geographical 

 limits which hinder free migration (e.g. high mountains, and seas in 

 the case of land plants and masses of land in the case of marine 

 organisms) stand in striking correspondence with differences in the 

 fauna and flora of particular habitats, countries, continents, or oceans. 

 The assemblages of organisms found in two continents differ as regards 

 their families, genera, etc., in proportion to the degree of present and 

 former isolation because the forms in each region have continued 

 their phylogenetic development independently. The easier the 

 exchange of forms between two regions the more numerous will be 

 those which are common to both. It is a general rule that the 

 inhabitants of any region are most closely related to those of the 

 nearest region from which migration may be assumed, on geological 

 and geographical reasons, to have taken place. This holds, for 

 example, for the Cape Verde Islands and the African mainland, and 

 for the Galapagos islands or Juan Fernandez and the neighbouring 

 regions of America. The more a habitat, such as an island, is isolated 

 from the rest of the world the richer will it tend to be in peculiar 

 forms (ENDEMISM) ; these often differ only slightly from other non- 

 endemic forms from which they have evidently originated, though 

 further dispersal has been impossible. 



4. PALAEONTOLOGICAL EVIDENCE. Palaeontology shows that in 

 the history of the earth species have become extinct and others 

 appeared ; that not infrequently the forms in successive geological 

 strata can be arranged in series showing progressive organisation ; 

 and that the groups which are regarded as most highly organised 

 appeared relatively late in the history of the earth (e.g. the Angio- 

 sperms in the Cretaceous period). It has also made us acquainted with 

 extinct intermediate types between genera, families, and classes. 

 That such cases are not more frequent evidently depends on the 

 incompleteness of the geological record. In Botany the most important 

 of these synthetic groups is that of the Pteridospermeae or Cycadofilices, 

 which are plants of the Carboniferous period connecting the Ferns 



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