xcvi REPORT — 1877. 



remarkable transformations of the vascular arches which proceed from the 

 aortic bulb along the several branchial arches, and which, in the gills of fishes 

 and aquatic Amphibia, undergo that minute subdivision which belongs to the 

 vascular distribution of gills, but which in the higher non-branchiated animals 

 are the subject of very different and various changes, in the partial obliteration 

 of some and the enlargement of others, by which the permanent vessels are 

 produced. 



These changes and transformations have for many years been a subject of 

 much interest to comparative anatomists, and will continue to be so, not only 

 from their presenting to us one of the most remarkable examples of confor- 

 mity in the plan of development and the type of permanent or completed 

 organization in the whole series of vertebrated animals, but also because of 

 the manifest dependence of the phenomena of their development upon ex- 

 ternal influences and atmospheric conditions affecting the respiration, nutri- 

 tion, and modes of life of the animal. 



Nor is the correspondence to which I now refer entirely limited to the 

 Vertebrata. For here, again, through the Amphioxus and the Ascidia, we 

 come to see how an affinity may be traced between organs of circulation and 

 respiration which at first appear to belong to very different types. The 

 heart of vertebrates is, as is well known, essentially a concentrated form of 

 vascular development in the ventral aspect of the body, while the heart of 

 the invertebrate, whether in the more concentrated form existing in the 

 Articulata and Mollusca or in a more subdivided shape prevalent in the 

 Annelida, is most frequently dorsal ; yet the main aorta of the Vertebrates 

 is also dorsal; and it is not impossible, through the intermediate form of 

 Amphioxus, to understand how the relation between the Vertebrate and the 

 Invertebrate type of the blood-vascular system may be maintained. 



But I am warned by the lapse of time that I must not attempt to pursue 

 these illustrations further. In the statement which I have made of some of 

 the more remarkable phenomena of organic production — too long, I fear, for 

 your endurance, but much too brief to do justice to the subject — it has been 

 my object mainly to show that they are all more or less closely related toge- 

 ther by a chain of similarity of a very marked and unmistakable character j 

 that in their simplest forms they are indeed, in so far as our powers of obser- 

 vation enable us to know them, identical ; that in the lower grades of animal 

 and vegetable life they are so similar as to pass by insensible gradations into 

 each other ; and that in the higher forms, while they diverge most widely in 

 some of their aspects in the bodies belonging to the two great kingdoms of 

 organic nature, and in the larger groups distinguishable within each of them, 

 yet it is still possible, from the fundamental similarity of the phenomena, to 

 trace in the transitional forms of all their varieties one great general plan of 

 organization. 



