390 THE EVOLUTION THEORY 



birds appear first in the Jurassic, but in a very different guise 

 (A)rh(xopteryx) from the modern forms, covered indeed with feathers, 

 but still possessing a reptilian tail ; later they occur as toothed birds 

 in the Cretaceous, and in Tertiary times they have their present form. 

 The development of mammals must have run almost parallel with 

 that of birds, that is, from the beginning of Secondary times onwards, 

 and their highest and last member appears, as far as is known to 

 research, only in post-Glacial times, in the Diluvial deposits. 



To the types which have arisen since the Cambrian period 

 belongs the class of Insects with its twelve orders and its enormous 

 wealth of known species, now reckoned at 200,000. Thej- are 

 demonsti'able first in the De\"onian, and then in the Carboniferous 

 period, in forms, just as our theory requires, with biting mouth- 

 organs ; it is not until the Cretaceous strata that insects with purely 

 suctorial mouth-organs — bees and butterflies — occur, as it was also at 

 that time that the flowers, which have evolved in mutual adaptation 

 with insects, flrst appeared. 



The number of fossil species hitherto described is reckoned at 

 about 8o,oco — certainly only a mere fragment of the wealth of forms 

 of life which have arisen on our earth throughout this long period, 

 and which must have passed away again ; for very few sjjecies outlive 

 a geological epoch, and even genera appear only for a longer or 

 shorter time, and then disappear for ever. But even of many of the 

 older classes, such, for instance, as the Cystoids among the Echino- 

 derms of the Silurian seas, no living representative remains ; and in 

 the same way, the Ichthyosaurs or flsh-lizards of the Secondary times 

 have completely disappeared from our modern fauna, and many other 

 animal types, like the class of Brachiopods and the hard-scaled Ganoid 

 fishes, have almost died out and are r-epresented only by a few 

 species in specially sheltered places, such as the great depths of the 

 sea, or in rivers. 



Thus an incredible wealth of animal and plant species was 

 potentially contained in these simplest and lowest ' Biophorids ' which 

 lay far below the limits of microscopic visibility — an indefinitely 

 greater wealth than has actually arisen, for that is only a small part 

 of what was possible, and of M'hat would ha^e arisen had the changes 

 of life-conditions and life-possibilities followed a different course. 

 The greater the complexity of the structure of an organism is, the 

 more numerous are the parts of it which are capable of variation, 

 and the different directions in which it can adapt itself to new con- 

 ditions; and it will hardly be disputed that potentially the first 

 Biophorids contained an absolutely inexhaustible wealth of forms 



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