388 GENERAL PRINCIPLES 



represented by fossil remains. Of these there are about 60,000 

 known. In addition to these there are many species living 

 which have not been observed by the recording biologist, and 

 hence do not appear in the count, and there have probably 

 been many, many more which became extinct without leaving 

 any trace or the remains of which have not yet been found. It 

 is, therefore, not probable that any one familiar with the facts 

 would regard a million a high estimate for the number of species 

 which have been or are now living on the earth. 



789. Origin of Species. — The question naturally arises, 

 Whence came they all? It is a question which has always 

 occupied thinking men, and concerning which there has been 

 much difference of opinion. To-day biologists generally, if 

 not all, are of the opinion that species are plastic, as it were, and 

 continually undergoing modification, so that they are not to-day 

 what they were or what they will be, and further that two sec- 

 tions of a species may become modified in different directions 

 and thus come to differ even to the extent of specific distinction. 

 In this way there would arise two species where there had been 

 but one. This is known as the theory of the origin of species 

 by descent with modification. In connection with this theory 

 there are two questions which should be clearly distinguished. 

 The first one is as to fact, the other as to method: (i) What 

 is the evidence that species originate by descent with modi- 

 fication? (2) If a fact, how does it come about? In the fol- 

 lowing pages we will consider the evidence upon which the 

 theory of descent rests, and in that connection take up a number 

 of important biological phenomena which have not yet been 

 discussed. 



790. The Taxonomic Series. — ^Long ago, students of natural 

 history were struck by the fact that animals could be arranged 

 in a series in the order of their various degrees of organization; 

 with the simplest at one end, the most complex at the other, and 

 the interval between more or less completely filled by forms of 



