MEMOIRS. 
Larval Forms : their Nature, Origin, and Affinities. 
By F. M. Balfour, M.A., F.R.S., Fellow of Trinity 
College, Cambridge. 
Preliminary considerations. — Animals either (1) undergo 
the whole of their early development in the egg or within 
the body of the parent, and are hatched in a condition closely 
resembling the adult ; or else (^) they are born in a condi- 
tion differing to a greater or less extent from the adult. 
When born in the latter condition they are known as 
larvae, till they have approximately acquired the adult 
characters of the species. There are no questions which are 
of greater importance for the embryologist than those which 
concern the nature of the secondary changes likely to occur 
in the foetal or in the larval state ; since it is on the answer 
to such questions that our knowledge of the extent to which 
a record of the ancestral history may be expected to be pre- 
served in development depends. The principles which govern 
the perpetuation of variations which occur in either the larval 
or the foetal state are the same as those for the adult state. 
Variations favorable to the survival of the species are equally 
likely to be perpetuated, at whatever period of life they occur, 
prior to the loss of the reproductive powers. The possible 
nature and extent of the secondary changes which may have 
occurred in the developmental history of forms, which have 
either a long larval existence, or which are born in a nearly 
complete condition, is primarily determined by the nature 
of the favorable variations which can occur in each case. 
\Vhere the development is a foetal one, the favorable 
variations which can most easily occur are — (1) abbrevia- 
tions, (2) an increase in the amount of food-yolk stored up for 
the use of the developing embryo. Abbreviations take place 
because direct development is always simpler, and therefore 
VOL. XX. NEW SER. C C 
