Where the development is a foetal one, the favourable variations which can most easily occur are--(1) abbreviations, (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 more advantageous; and, owing to the fact of the foetus not being required to lead an independent existence till birth, and of its being in the meantime nourished by food-yolk, or directly by the parent, there are no physiological causes to prevent the characters of any stage of the development, _which are of functional importance during a free but not during a foetal existence_, from disappearing from the developmental history. All organs of locomotion and nutrition not required by the adult will, for this reason, obviously have a tendency to disappear or to be reduced in foetal developments; and a little consideration will shew that the ancestral stages in the development of the nervous and muscular systems, organs of sense, and digestive system will be liable to drop out or be modified, _when a simplification can thereby be effected_. The circulatory and excretory systems will not be modified to the same extent, because both of them are usually functional during foetal life. The mechanical effects of food-yolk are very considerable, and numerous instances of its influence will be found in the earlier chapters of this work[135]. It mainly affects the early stages of development, _i.e._ the form of the gastrula, &c. [135] For numerous instances of this kind, _vide_ Chapter XI. of Vol. III. The favourable variations which may occur in the free larva are much less limited than those which can occur in the foetus. Secondary characters are therefore very numerous in larvæ, and there may even be larvæ with secondary characters only, as, for instance, the larvæ of Insects. In spite of the liability of larvæ to acquire secondary characters, there is a powerful counterbalancing influence tending towards the preservation of ancestral characters, in that larvæ are necessarily compelled at all stages of their growth to retain _in a functional state_ such systems of organs, at any rate, as are essential for a free and independent existence. It thus comes about that, in spite of the many causes tending to produce secondary changes in larvæ, there is always a better chance of larvæ repeating, in an unabbreviated form, their ancestral history, than is the case with embryos, which undergo their development within the egg. It may be further noted as a fact which favours the relative retention by larvæ of ancestral characters, that a secondary larval stage is less likely to be repeated in development than an ancestral stage, because there is always a strong tendency for the former, which is a secondarily intercalated link in the chain of development, to drop out by the occurrence of a _reversion_ to the original type of development.