612 ZOOLOGY SECT. 



In certain cases among the Mammalia a number of closely 

 related stages have been discovered, showing, taken in their 

 chronological order, a gradually increasing specialisation of struc- 

 ture. One of the best known examples of this is that of the 

 Horse, to which attention is directed in the section on the 

 Mammalia (p. 572). And there are other families of mammals, 

 chiefly among the Ungulates (the family of the Pigs and various 

 families of Ruminants) in which an equally complete history has 

 been made out. 



The direct evidence of the evolution of the Invertebrates is, in 

 general, very imperfect. Some existing types of a comparatively 

 highly-organised character are to be recognised among the fossil 

 remains in the oldest formations the Cambrian in which definite 

 organic structures, if we except a few Radiolaria and Foraminifera, 

 are traceable. There is no trace of primitive fossil members of 

 the various invertebrate phyla, and the highly organised air- 

 breathing Arthropods are represented both by Scorpions and by 

 Insects as far back as the Silurian. Such remarkably complete 

 geological histories as have been traced in some of the Mammalia 

 are extremely rare in the Invertebrates. Such direct evidence, 

 however, as is obtainable, points to the probability of evolution, 

 and it may be inferred that the absence of primitive generalised 

 representatives of the invertebrate phyla is most probably due 

 to the imperfect character of the geological record. 



The Lamarckian Theory. Supposing it to be regarded as 

 proved that the organic world has come to be as we find it by a 

 process of gradual evolution, we have next to inquire by what 

 agencies this process of development has been brought about. 



A sketch of the history of thought on this subject will be 

 given in the section on the history of Zoology, and it will not be 

 necessary here to refer to more than one or two names. The 

 first important attempt to solve the problem regarding the 

 nature of the forces by means of which evolution has taken place, 

 was made, long before evolution was generally accepted among 

 men of science, by Lamarck in his Pliilosopldz Zooloyique, published 

 in 1809. Lamarck's view was that evolution of new forms has 

 taken, and is taking, place, in great measure owing to the direct 

 action of the conditions of life on the organism, but still more 

 owing to the use and disuse of organs. The surroundings or 

 environment of the animal or plant produce a direct effect on the 

 individual bring about slight modifications in one direction or 

 another, and these slight differences are transmitted by inherit- 

 ance to the next generation such slight modifications going on 

 generation after generation, producing eventually a marked effect 

 on the characters of the organism. The chief agencies that might 

 be supposed to act in this way are climate, the nature of the 

 country, and food. But, in addition to these, Lamarck attributes 



