146 
ANIMAL PEDIGREES. 
July. 1891. 
of say the fourth day, is clearly not an animal capable of 
independent existence, and therefore cannot correctly repre¬ 
sent 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 embryologists ; and the attempt 
to determine them has as yet met with only partial success. 
The most potent and the most widely spread of these dis¬ 
turbing causes arise from the necessity of supplying the 
embryo with nutriment. This acts in two ways. If the 
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 sufficient 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 food, it commonly happens that these 
stages are passed through in a very modified form, the 
embryo being, as in a four-day chick, in a condition in which 
it is clearly incapable of independent existence. 
The nutrition of the embryo prior to hatching is most 
usually effected by granules of nutrient matter, known as 
food yolk, aud embedded in the protoplasm of the egg itself; 
and it is on the relative abundance of these granules that the 
size of the egg chiefly depends. 
Large size of eggs implies diminution of the number of 
the eggs, and hence of the offspring; and it can be well 
understood that while some species derive advantage in the 
