62 Thomas Henry Huxley 



combine with another chemical substance only in a 

 fixed proportion is necessary and inherent in the exist- 

 ing conception of it. There was no ghmmer of the 

 idea that these types were not inherent, but merely 

 historical results of a long and slow series of changes 

 produced by the interaction of the varied conditions of 

 life and the intrinsic qualities of living material. 



In two lectures delivered at the Royal Institution in 

 1854 and 1855, the one on " The Common Plan of Ani- 

 mal Forms," the other on " The Zoological Arguments 

 Adduced in Favour of the Progressive Development of 

 Animal Life in Time," show, so far as the published 

 abstracts go, the same condition of mind. The idea of 

 progressive development of all life from common forms 

 was not unknown to Huxley and his contemporaries, 

 but was rejected by them. In the first of these two 

 lectures he took four great groups of animals, the Verte- 

 brates, the Articulata, the Mollusca, and the Radiata, 

 and explained what was the archetype of each. He 

 shewed the distinctiveness of each plan of structure, and 

 then discussed the relations of the ideas suggested by 

 Von Baer to these archetypes. He stated explicitly 

 that while the adult forms were quite unlike one an- 

 other, there were traces of a common plan to be derived 

 from a stud}^ of their embryonic development. Such a 

 trace of a common plan he had himself suggested when 

 he compared the foundation-membranes of the Medusae 

 with the first foundation-membranes of vertebrate em- 

 bryos. This was going a long way towards modern 

 ideas ; but he stopped short, and gav^e no hint that he 

 believed in the possibility of the development of one 

 plan from a lower or simpler plan. The second lecture 

 dealt with the kind of ideas which were crystallised in 

 the popular but striking work of Chambers, entitled 



