EVOLUTION, VARIATION AND HEREDITY 447 



plants too for that matter, are not simply isolated forms unrelated 

 to one another, but can be arranged in groups, showing marked 

 similarity among themselves. The members of the smaller groups, 

 the genera, are very closely related and often differ mainly in size, 

 proportion of parts, and colour. Consider, for example, the cat family, 

 the Felidce, we have Felis leo the lion, F. tigris the tiger, F. pardus 

 the leopard, F f lynx the lynx, F. concolor the puma, F. domesticus the 

 domestic cat, and other less known species. Every one is sufficiently 

 familiar with the appearance of all these to recognise that the differ- 

 ences between them are mainly those of size, colour, markings, and 

 proportion of parts. They are, however, all obviously similar 

 animals, and indeed in a standard text-book of mammalian anatomy 

 we read, " The Tiger (F. tigris) is so closely related to the Lion that it 

 is chiefly by external characters that the two species are distin- 

 guished." This likeness is most reasonably explained by supposing 

 them all to be the modified descendants of one original distant 

 ancestral " cat." Larger groups, e.g. Amphibia, Reptilia, Aves, 

 and Mammalia, although differing enormously in many respects, are 

 very obviously built on the same general plan. They have a skeleton 

 consisting of : a vertebral column, i.e. a number of essentially similar 

 bones in a series forming the main support of the body, and a canal 

 for the reception of the spinal cord ; a skull, i.e. a collection of bones 

 giving lodgment to the brain, the olfactory optic and auditory 

 organs, and furnishing a pair of jaws ; pectoral and pelvic girdles, 

 giving support to paired pentadactyl limbs, and so on with the other 

 systems. All of them exhibit the same basal plan, and where 

 differences occur we can see very often that the change enables the 

 animal to be better suited to its environment. The most obvious 

 explanation of this is to suppose that they have all descended from an 

 ancestral form that exhibited the general plan in a simple unmodified 

 condition. Similar evidence confronts us in every group, large or 

 small, in the animal and vegetable kingdoms. Why is it that our 

 own fore limb, the fore limbs of a cat, a whale, a bat, and a bird are 

 all composed of essentially similar parts although adapted for quite 

 different purposes ? The simplest answer is that they all represent 

 modifications of one and the same thing. 



We saw in Scyllium that the ear is composed of a complex mem- 

 branous labyrinth, and lies near the first gill cleft, and is concerned 

 largely with equilibration. In the frog, which no longer needs a 

 gill cleft, since it is an air-breathing form, the outside of the cleft is 

 covered by a tightly stretched membrane, the tympanum, which 

 becomes accessory to the ear. The old ear, as in Scyllium, is still 

 present though more developed, but hearing becomes a more im- 

 portant function, and for increased efficiency further structures, the 



