xxvi INTRODUCTION 



small collateral branches : a characteristic of this series 

 would be that each species would differ from the neighbouring 

 species by gradations so small as to be imperceptible. From 

 species to species, however, organisation gradually develops, 

 so that there are conspicuous differences discernible when 

 we compare two animals situated at some little distance 

 from one another in the scale of nature. The scale begins 

 with Monas and ends with Man : these two infinitely different 

 creatures being connected by the entire series of animal 

 sp&cies, each one differing from its neighbours only in minute 

 details. Lamarck attributed the large gaps which appear 

 in various parts of the scale to undiscovered species ; for, 

 as we shall shortly see, he did not believe that species had 

 ever become extinct. Between the various orders of birds 

 or mammals, for instance, there often seems no obvious 

 connecting link : and the gap between these two classes 

 themselves is a wide and apparently impassible one. The 

 discovery at the end of the eighteenth century of Ornitho- 

 rhyncus and Echidna, which possess many of the characters 

 both of mammals and birds, naturally appeared to Lamarck 

 strong confirmation of his theory that the gaps existing in 

 the linear series of animals, merely represent the existence 

 of living species, hitherto undiscovered, either because 

 they belong to unexplored lands or to the bottom of the 

 ocean. Lamarck's scale of nature was thus to a certain 

 extent analogous with the modern periodic classification of 

 the elements. In each case, gaps occur which are liable, 

 as knowledge advances, to be filled up by new species or 

 new elements, possessing properties that can be prophesied 

 in advance. 



If there is in reality no hiatus or gap in the progressive 

 Hnear series of animals, then, argued Lamarck, there cannot 

 be any isolated groups of animals, nor is there any objective 

 justification for breaking up the animal kingdom into classes, 

 orders, families, genera, etc. In fact, all such classifications 

 must be purely arbitrary. They may indeed set forth any 

 portion of the animal series, arranging the species in their 



