832 REPORT—1890. 
It is owing to this Recapitulation Theory that Embryology has exercised so 
marked an influence on zoological speculation. Thus the formation in most, if 
not in all, animals of the nervous system and of the sense organs from the 
epidermal layer of the skin, acquired a new significance when it was recognised 
that this mode of development was to be regarded as a repetition of the primitive 
mode of formation of such organs; while the vertebral theory of the skull 
affords a good example of a view, once stoutly maintained, which received its 
death-blow through the failure of embryology to supply the evidence requisite in 
its behalf. The necessary limits of time and space forbid that I should attempt to 
refer to even the more important of the numerous recent discoveries in embryology, 
but mention may be very properly made here of Sedgwick’s determination of 
the mode of development of the body cavity in Peripatus, a discovery which has 
thrown most welcome light on what was previously a great morphological puzzle. 
We must now turn to another side of the question. Although it is undoubtedly 
true that development is to be regarded as a recapitulation of ancestral phases, 
and that the embryonic history of an animal presents to us a record of the race 
history, yet it is also an undoubted fact, recognised by all writers on embryology, 
that the record so obtained is neither a complete nor a straightforward one. 
It is indeed a history, but a history of which entire chapters are lost, while in those 
that remain many pages are misplaced and others are so blurred as to be illegible ; 
words, sentences, or entire paragraphs are omitted, and worse still, alterations or 
spurious additions have been freely introduced by later hands, and at times so 
cunningly as to defy detection. 
Very slight consideration will show that development cannot in all cases be 
strictly a recapitulation of ancestral stages, It is well known that closely allied 
animals may differ markedly in their mode of development. The common frog is 
at first a tadpole, breathing by gills, a stage which is entirely omitted by the 
‘West Indian Hylodes. A crayfish, a lobster, and a prawn are allied animals, 
yet they leave the egg in totally different forms. Some developmental stages, as 
the pupa condition of insects, or the stage in the development of a dogfish in 
which the cesophagus is imperforate, cannot possibly be ancestral stages. Or again, 
a chick embryo of say the fourth day is clearly not an animal capable of inde- 
pendent existence, and therefore cannot correctly represent any ancestral condition, 
an objection which applies to the developmental history of many, perhaps of most 
animals. 
Haeckel long ago urged the necessity of distinguishing in actual development 
between those characters which are really historical and inherited and those 
which are acquired or spurious additions to the record. The former he termed 
palingenetic or ancestral characters, the latter cenogenetic or acquired. The 
distinction is undoubtedly a true one, but an exceedingly difficult one to draw in 
practice. The causes which prevent development from being a strict recapitulation 
of ancestral characters, the mode in which these came about, and the influence 
which they respectively exert, are matters which are greatly exercising embryolo- 
gists, and the attempt to determine which has as yet met with only partial success. 
The most potent and the most widely spread of these disturbing causes arise 
from the necessity of supplying the embryo with nutriment. This acts in two 
ways. Ifthe amount of nutritive matter within the egg is small, then the young 
animal must hatch early, and in a condition in which it is able to obtain food for 
itself. In such cases there is of necessity a long period of larval life, during which 
natural selection may act so as to introduce modifications of the ancestral history, 
spurious additions to the text. 
If, on the other hand, the egg contain within itself a considerable quantity of 
nutrient matter, then the period of hatching can be postponed until this 
nutrient matter has been used up. The consequence is that the embryo hatches at 
a much later stage of its development, and if the amount of food material is suffi- 
cient may even leave the egg in the form of the parent. In such cases the earlier 
developmental phases are often greatly condensed and abbreviated; and as the 
embryo does not lead a free existence, and has no need to exert itself to obtain 
