Lefevre — The Advance of Zoology in the Nineteenth Century. 83 



for the rest, including Crustacea, Insects, Myriapods and 

 Spiders. 



We now arrive at the Darwinian Era, or the period when 

 the doctrine of organic evolution became established. Al- 

 though I shall speak of the development of evolutionary 

 views in another place, it will be well here to refer to the pro- 

 found influence which the acceptance of the theory of natural 

 selection exerted upon the progress of morphology. With 

 the rejection of the old and erroneous conception of species 

 and with the establishment of the doctrine of genetic descent 

 of all living things, at once a natural classification of animals 

 became possible, a classification which should express, not 

 arbitrarily chosen differences and resemblances, but actual 

 relationships. The nearness or remoteness of descent of a 

 given form would now determine its position in the system, 

 which would thus be an attempt to indicate the lines of descent 

 and interrelationships of all known animals. 



A flood of light burst for the first time on the mass of 

 accumulated facts which gradually began to assume an 

 orderly arrangement and to take their proper positions ac- 

 cording to the general principle of organic evolution. Facts 

 previously misinterpreted received a rational explanation, and 

 facts which before had no significance, or which had merely 

 been referred to the will or pleasure of a Creator, now as- 

 sumed a real meaning. The breath of life as it were had 

 been breathed into the science of zoology. 



Evidence for the theory of descent is drawn from four 

 great sources, namely, comparative anatomy, embryology, 

 palaeontology and geographical distribution ; and it is but 

 natural that with the acceptance of the doctrine, which soon 

 became practically universal among zoologists, these four de- 

 partments of zoological investigation should have sprung for- 

 ward with giant leaps in the feverish haste of workers to gain 

 further evidence for Darwinism. Homology had received a 

 real explanation, for the reason why a part of one animal re- 

 sembles in structure the part of another, though perhaps dif- 

 fering in function and external appearance, is because it has 

 been inherited in both cases from an ancestor possessing a 

 similar part constructed after the same fundamental pattern. 



