52 General Part. 



As a matter of fact, these changes also take some time, and the 

 development only appears to be a sudden one. 



It is frequently found that an animal, during its development, wietlier in. 

 the egg or in the larval state, shows certain characters, wanting in the adult, 

 which are proper to a lower type. Tadpoles, for instance, possess gills, and 

 a piscine arrangement of the greater part of the hlood vascular system. Many 

 lai-val Decapoda have an exopod upon the last pair of thoracic legs, which 

 atrophies later, but is permanent in many lower Crustacea. Among the higher 

 Vertebrates, gill-slits, etc., are present in the embryo. Frequently, as in the last, 

 mentioned, these structures are funotionless, and of a rudimentary nature. 

 These facts are only intelligible when it is admitted that the forms in question 

 were derived from, the lower type, or from one near it, and that these particular 

 characters are retained in an early stage of development, though lost in the adult 

 (see next Section). 



Parental Instinct. — In many cases the egg or the embryo after 

 leaving the body of the parent is specially cared for by the mother, or 

 more rarely, by the father. The object of this is usually two-fold,, 

 firstly, to protect the egg or young animal from dangers of all 

 sorts ; secondly, to supply it v\rith food. Less frequently some special 

 means is resorted to, in order to keep the egg or young animal at the 

 temperature most favourable to development (Birds). In the simplest 

 cases the mother is satisfied with depositing the egg in a safe place,. 

 [e.g., by burying it), or parental solicitude is confined to the careful 

 selection of the place in which the eggs are to be laid, so that the- 

 young ones may at once find suitable food. In other cases, the 

 mother sits upon the eggs until the young animals hatch out, or even 

 after this, in order to protect and warm them : or the eggs and young 

 ones are carried about by her. The parental instinct is still 

 more actively manifested in cases where the young ones are able ■ to 

 take in and digest food, but are still unable to procure it for themselves, 

 and are therefore fed for some time by the mother. In many instances^ 

 the care of the brood not only dominates the whole life of the parent, 

 but may even lead to the formation of special organs, or to the- 

 modification of those already existing, (brood sac, mammary glands, 

 etc.) ; in many cases, too, it closely affects the mode of development 

 both of the eggs and young animals, many features of which appear- 

 to be largely determined by this factor. [See, for example, the 

 Mysidae, the Marsupials.) 



Careful consideration leads to the belief that the -viviparous method of 

 retaining the egg -^vithin the body of the mother is only a form of parental care. 

 The difference between providing for the embryo -within the o-viduot (as in 

 all -viviparous animals), or in a pouch of the slrin (Marsupials), appears to 

 be a purely superficial one ; the aim is in both cases identical, and in both cases, 

 the results are the same for the parent as well as for the young animal ; the 

 formation of some special arrangement in the former, some peculiarities in the 

 development of the latter. 



