194 JOURNAL OF THE PLYMOUTH INSTITUTION. 
the expansion and contraction of these orifices, and spending its 
whole life rooted to the rock or shell as firmly as the plant is 
rooted to the earth. 
8. A third illustration is drawn from the similarity of plan 
which may be traced in the region of the face and neck, including 
the apparatus of the jaws and gills. 
4. A fourth from the history of the development of the first pair 
of arches, which ‘‘ includes the basis of the formation of the lower 
jaw with the so-called cartilage of Mcchel, and which, while 
furnishing the bone which suspends the lower jaw in reptiles and 
birds, is converted in mammals into the hammer-bone of the ear.” 
5. A fifth illustration is found in the comparative anatomy of 
the heart and its mode of formation in the embryo, which furnishes 
most striking illustrations of the relation between entogenetic and 
phylogenetic development in the vertebrates, and is not without its 
application to some of the invertebrates. 
The list is closed by a slight reference to the malformation to 
which the heart is subject, and due to the persistence of transitory 
conditions which belong to different stages of progress in the 
development of the embryo, and a hasty glance at the affinity 
which may be traced between organs of circulation and respiration 
which at first appear to belong to very different types, but which 
may be identified both in vertebrates and, through the AmBIIGED 
and Ascidian, in the invertebrates also. 
As the result of this process of investigation and comparison, 
Dr. Thompson is convinced that, ‘‘the phenomena which have been 
ascertained as to the first origin and formation of textures and 
organs in any individual animal are of so uniform a character as to 
indicate forcibly a law of connexion and continuity between them;” 
and he is ‘‘equally convinced of the similarity of plan in the 
development of the larger groups, and to some extent of the whole.” 
He considers it therefore ‘‘impossible for any one to be a faithful 
‘student of embryology, in the present state of science, without at 
the same time becoming an Evolutionist,” and regards it as “no 
exaggerated representation of the present state of our knowledge 
to say, that the ontogenetic development of the individual in the 
higher animals repeats, in its more general character and in many 
of its specific phenomena, the phylogenetic development of the race.” 
But surely another conclusion may with equal reason and as great 
fairness be drawn from the phenomena of embryology which Dr. 
