Professor Owen's Address. 383 



mental change; and Zootomy, Histology and Embryology combine 

 tbeir results in forming an adequate and lasting basis for the 

 higher axioms and generalizations of Zoology properly so called. 

 Three principles, of the common ground of which we may ulti- 

 mately obtain a clearer insight, are now recognized to have 

 governed the construction of animals : — unity of plan, vegetative 

 repetition, and fitness for purpose. The independent series of 

 researches by which students of the articulate animals have seen, 

 in the organs performing the functions of jaws and limbs of varied 

 powers, the same or homotypal elements of a series of like segments 

 constituting the entire body, and by which students of the verte- 

 brate animals have been led to the conclusion, that the maxillary, 

 mandibular, hyoid, scapular, costal and pelvic arches, and their 

 appendages sometimes forming limbs of varied powers, are also 

 modified elements of a series of essentially similar vetebral seg- 

 ments, — mutually corroborate their respective conclusions. It is 

 not probable that a principle which is true for Articulata should 

 be false for Vertebrata : the less probable since the determination 

 of homologous parts becomes the more possible and sure in the 

 ratio of the perfection of the organization. 



MICROSCOPIC INVESTIGATIONS. 



The microscope is an indispensable instrument in embryological 

 and histological researches, as also in reference to that vast swarm 

 of animalcules which are too minute for ordinary vision. I can 

 here do little more than allude to the systematic direction now given 

 to the application of the microscope to particular tissues and parti- 

 cular classes chiefly due, in this country, to the counsels and exam- 

 ple of the Microscopical Society of London. A very interesting ap- 

 plication of the microscope has been made to the particles of matter 

 suspended in the atmosphere; and a systematic continuation of such 

 observations by means of glass slides prepared to catch and retain 

 atmospheric atoms, promises to be productive of important results. 

 We now know that the so-called red-snow of Arctic and Alpine re- 

 gions is a microscopic single-celled organism which vegetates on the 

 surface of snow. Cloudy or misty extents of dust-like matter per- 

 vading the atmosphere, such as have attracted the attention of 

 travellers in the vast coniferous forests of North America, and have 

 been borne out to sea, have been found to consist of the "pollen" 

 or fertilizing particles of plants, and have been called "pollen 

 showers." M. Daneste, submitting to microscopic examination 



