6 COMPAEATIVE ANATOMY. 



§ 5. 



By means of inheritance, characters are passed on to the 

 organism, which are afterwards matured in the course of its indi- 

 vidual development (Ontogeny). There is no such development in 

 the simplest forms, inasmuch as the young, which arise by division of 

 the maternal organism, only need increase in size to make them like 

 the maternal organism. In this case, development is the same thing 

 as growth, which is completely coextensive with it. The farther 

 an organism is from a primitively simple condition, or the greater 

 the sum of characters which have been inherited from its ancestors 

 and transmitted to their descendants, the less simple is its ontogeny ; 

 for during it a part at least of the characters which have been 

 inherited from its ancestors are repeated, and are presented by the 

 developing body in several successive stages. Ontogeny thus 

 represents, to a certain degree, palaaontological development, ab- 

 breviated or epitomised. The stages which are passed through by 

 higher organisms in their ontogeny, correspond to stages which 

 are maintained in others as the definitive organisation. These 

 embryonic stages may accordingly be explained by comparing them 

 with the mature stages of lower organisms, since we regard them as 

 forms inherited from ancestoi'S belonging to such lower stages. 

 Regarded from this point of view, many of the so-called larval- 

 stages, with their " provisional organs" — so named because they are 

 transitory, and limited to the earlier stages of life only — are seen 

 to be forms of great importance, and full of significance. Such 

 organs, besides having physiological relations to the organism 

 which possesses them, in consequence of which they are preserved 

 as practical arrangements, and become heritable, can be recog- 

 nised in lower grades of the existing series of animal forms, and 

 thus reveal the phylogeny of the animal that possesses them. The 

 "stadium larvatum" then, notwithstanding its name, often points 

 out with great clearness the blood-relationship of an organism. At 

 times these larval organs are not so well explained by transmission 

 as by adaptation, and thus the estimation of their true meaning is 

 made more difficult. The significance of these arrangements is more 

 obvious in organisms which do not enter immediately into the 

 " struggle for existence " in the external world, but are developed for 

 a certain time within the coverings of the ovum, and so are less 

 exposed to the moulding influences of the outer world. In these 

 cases they are " provisional " arrangements, and may be with greater 

 . certainty regarded as having been transmitted, and consequently as 

 repetitions of lower stages. The branchial clefts which appear in 



