LECTUEE II 

 THE DARWINIAN THEORY 



Period of detailed research — Appearance of Darwin's Origin of Species — Darwin's 

 life — Voyage round the world — His teaching — Domesticated animals, dog, horse — 

 Pigeons — Artificial selection — Unconscious selection — Correlated variations. 



The period of wholly unphilosophical, purely detailed research 

 may be reckoned as from about 1830 to i860, though, of course, 

 many of the labours of the earlier part of the century must be 

 counted among the investigations which were carried out without 

 any reference to general questions, and even after 1H60 numerous 

 such works appeared. Nor could it be otherwise, for the basis of 

 all science must be found in facts, and the thorough working up 

 of the fact-material will always remain the first and most indis- 

 pensable condition of our scientific progress. During the period 

 referred to, however, it had become the sole end to be striven for; 

 and all energies were concentrated exclusively on the accumulation 

 of facts. 



The previous century had added much to the knowledge of the 

 inner structure of animals, the so-called ' comparative anatomy,' and 

 in the nineteenth century this line of investigation was pursued even 

 more extensively and energetically, so that the knowledge increased 

 enormously. Up till this time it was chiefly the structure of the 

 backboned animals and of a few ' backboneless ' animals, so called, 

 that had been studied, but now all the lower groups of the animal 

 kingdom were also investigated, and became known better and in 

 more detail as the methods of research improved. 



Not content, however, with a knowledge of the adult animal, 

 naturalists began to investigate its development. In the year 

 1 814 the first great work on development appeared, on the develop- 

 ment of the chick, by Pander and Von Baer. It was there shown 

 for the first time, how the chick begins as a little disk-shaped 

 membrane on the surface of the yolk of the egg, at first simply as 

 a pale streak, the 'primitive streak,' then as a groove, the 'primitive 

 groove,' by the side of which arise two folds, the ' medullary folds,' 

 and further how a system of blood-vessels is developed around this 

 primitive rudiment on the upper surface of the yolk, how a heart 



