Natural Selection: the keynote of the Darwinian Theory. The Changes in environment. Geological evidence of changes of climate Comparison between Natural and Artificial Selection. Natural THE ARGUMENT FROM PALÆONTOLOGY. The Structure of the Earth's Crust. A series of superposed layers or strata, composed of various materials, originally deposited for the most part under water, and subsequently raised above the Fossils are the actual remains, as bones, teeth, shells, etc., or the indications, as, e.g., footprints, of the animals that formerly in- habited the earth. Amongst them, were our collections com- plete, would be the entire series of the ancestors of all living animals. The actual age of Fossils cannot be determined, but their relative ages are known from the position of the strata in The Imperfection of the Geological Record. Only certain parts of The Geological Evidences of Evolution. Life on the earth has been The Extinction of Species. Persistent types. Comparison of the THE GEOGRAPHICAL DISTRIBUTION OF ANIMALS. The Problems of Geographical Distribution; and the methods of The Geographical Distribution of Camels, Marsupials, and Tapirs. THE ARGUMENT FROM EMBRYOLOGY. Embryology as a clue to Zoological affinities. The development of an The Recapitulation Theory. The doctrine of Evolution tells us that Illustrations of Recapitulation. The development of Flat fish, Crabs, Prawns and Barnacles. Embryology and Paleontology. Examples of Recapitulation as seen Rudimentary or Vestigial Organs: structures which are present in a condition in which they can be of no use to their possessors. Natural Selection will not account for the formation or perpetuation of such structures; but the Recapitulation Theory explains these at once, as organs which were of functional value to the ancestors of their present possessors, and which appear in the development of existing forms owing to the tendency to repeat ancestral characters. Examples of Vestigial Organs: their Zoological importance. Causes tending to falsify the Ancestral History as preserved in actual development. 1. The tendency to condensation of the Ancestral History. 2. The tendency to the omission of Ancestral Stages. The structure of an egg. Germ yolk and food yolk. The causes 3. The tendency to distortion of Ancestral Stages. 4. The tendency to the accentuation, or undue prolongation of Particular Stages; best seen in cases of abrupt transformation or 5. The tendency to the acquisition of new characters, through the Tests of Recapitulation. All stages must be possible ones, and each stage must be an improvement on the preceding one. The eyes Embryonic Stages regarded as Ancestral Forms. The egg is the LECTURE V. (Pp. 116-150.) THE COLOURS OF ANIMALS AND OF PLANTS. Colour may be non-significant, as in the case of the redness of the Of late years our knowledge on this subject has advanced greatly, mainly through the observations of Mr. Wallace, sup- plemented by those of Mr. Bates, Mr. Trimen, Mr. Poulton, and others. COLOURS OF ANIMALS. 1. Apatetic Coloration. Apatetic Coloration serves to hinder recognition: it may be considered under three heads :- (a) Protective resemblances : aiding escape from enemies. The resemblances are usually either to plants, as in the case of the A peculiar and interesting form of protective resemblance is (b) Aggressive resemblances are cases in which the object objects. Examples are afforded by the whiteness of the Polar (c) Alluring resemblances are cases in which the coloration is Sematic Coloration is the direct opposite of Apatetic Colora- tion, and aims at securing recognition; it is of two chief (a) Warning Colours. Insects or other animals which are inedible owing to an unpleasant flavour or other cause, are usually very conspicuously coloured: the object being to adver- tise their inedibility, and to secure instant recognition, lest they should be killed by mistake. (b) Recognition Markings, such as the white tail of the Rabbit and the markings on certain Deer, are believed to aid recogni- Under this head all the cases of Sexual Coloration are included, in which, as in the Peacock, the bright colouring is The bright colours of Flowers and of Fruits serve to attract Cross-Fertilisation. The methods of ensuring cross-fertilisation in "Natural Selection tends only to make each organic being as per- "Natural Selection cannot possibly produce any modification in a Missing Links. Erroneous ideas as to the true nature of " links," Persistent Types. Many instances are known of genera of animals which have persisted, without appreciable modifications in structure, for enormously long periods, and in some cases from Silurian times to the present day. The occurrence of such persistent types is in no way opposed to the Theory of Natural Degeneration or Retrograde Development. An animal may be less Difficulty as to the persistence of lowly organised animals alongside the Alleged uselessness of small variations. The whole theory and practice of breeding domestic animals depend on selecting the right animals by scrupulous attention to minute differences. The objection that the right variation may not be present is met by the fact that variation affects all organs and occurs in all Difficulty as to the earliest commencement of organs. Natural Selection can only act on an organ after it has already attained sufficient size to be of practical importance and utility. The Theory of Change of Function: an organ may lose its original purpose and yet persist because it is of use for some other purpose: one of |