SOME TEACHINGS OF DEVELOPMENT. 
217 
represented by transitory or even by permanent conditions of the 
nervous system of a series of animals lower in the scale of organi- 
sation. If we were to analyse in the same way the development 
of any other specialised segregation of one of the higher animals, 
we should find that exactly similar results would be arrived at. 
Did time and space allow, it would be easy to trace the corre- 
spondences of development in the case of the heart, of the central 
axis or notochord, of the branchial slits, of the renal organs, and 
so on. And if this correspondence of permanent conditions in 
lower animals, with stages of development in higher animals, 
is thus found to obtain to minute detail in the separate 
and specialised parts, it stands to reason that it must be 
found also in the aggregate which those parts compose. And 
indeed, in many cases, even in the mere matter of external form, 
the correspondence is often such as to strike the most ordinary 
observer. Compare, for example, the polyp stage of development 
of the jelly-fish with the permanent conditions of some of the 
Hydroid polyps ; compare the Scyphistoma stage of Aurelia with 
the permanent condition of Lucernaria ; compare the tailed larva 
of Phallusia with the permanent condition of Appendicularia ; 
compare the several stages of transformation of one of the higher 
Crustacea with permanent conditions of the lower Crustacea ; 
compare the various stages of the developing Amphibian with 
either transitory or permanent conditions of AYorms, of Tunicates, 
of Amphioxus, and of lower Yertebrates ! 
In conclusion I will attempt to formulate as briefly as pos- 
sible, some of the general results at which we are able to arrive 
from a consideration of the facts we have had before us : 
I. If we compare the processes of development of any two 
animals, from Sponges upwards, we find complete correspondence 
up to a certain point ; from which point they may diverge from 
one another. This point is sometimes placed near the bottom of 
the development-scale, sometimes near the top ; or, it may be, in 
any intermediate position. 
II. Development is essentially localisation of function and 
concomitant or consequent modification of structure ; such modi- 
fication being accompanied by segregation of the cells concerned 
with the function localised. 
III. The path of development of all the more important of 
these segregated parts is the same up to a certain point in the 
development of each segregation. Trom this point it may, in any 
animals or group of animals, diverge from the rest ; or may remain 
stationary, whilst in the others, specialisation and modification 
progress further. 
IV. The various stages or phases of development of an animal, 
as well as of its specialised parts, are often found to correspond 
