978 



SCIENCE. 



[N. S. Vol. XV. No. 390. 



they have no air saes at all and their lungs 

 are free in the pleural sacs; they are 

 utterly devoid of a crop, and gizzard and 

 stomach are but indistinctly delimited 

 from one another, a condition ephemeral 

 with us. The nails of most of them are 

 clumsy and broad, as with us before hatch- 

 ing. Of all of them only the bats, which 

 seem to be the most highly developed, 

 possess the ability to fly. And these mam- 

 mals, who for so long a time after birth are 

 utterly helpless, and who during their 

 whole lives can never raise themselves off 

 the ground, claim to be more highly organ- 

 ized than we. 



(&) If it were a law of nature that the 

 development of an individual consists in 

 passing through the adult stages of ani- 

 mals less highly developed, it would follow : 



1. That no embryo could pass through 

 stages which do not characterize the adult 

 condition of some animal. There are no 

 animals, however, which carry their food 

 around in a yolk sac, and yet from the de- 

 velopment of birds and certain mammals, 

 such animals ought, according to the law, 

 to exist. 



2. Just as the environment of an embryo 

 is related to the presence of organs which 

 occur in no higher forms, so it makes im- 

 possible the passage through certain lower 

 stages. Thus since all the higher embryos 

 are bathed in water, that distinctive char- 

 acteristic of insects, the trachea;, can never 

 develop. 



3. An embryo, according to the prevail- 

 ing theory, should resemble in its various 

 stages a lower form, not merely in one 

 particular, but in all. If at the time when 

 the chambers of the heart are not yet 

 separate, and the digits have not yet be- 

 come distinct, the embryo is said to be in 

 the fish stage, where is the flattened tail 

 and all that makes up a fish and appears 

 so early in its development? 



4. There should be no ephemeral organs 



in lower animals which are permanent in 

 higher ones, but there are many such, to 

 some of which the bird embryologist has 

 already called attention. 



5. The organs in the different classes of 

 aninials should appear in the same condi- 

 tion in which we find them during the em- 

 bryonic life of higher ones, but this is 

 scarcely ever so. 



6. Those structures found only in higher 

 animals should appear late in their de- 

 velopment. This, however, is by no means 

 true. Parts of the vertebral column and 

 the arches of the vertebrje appear in the 

 chick earlier than any other organs. How 

 can the chick ever resemble an inverte- 

 brate 1. 



III. 



THE RELATIONSHIPS BETWEEN DIFFERENT 

 ADULT ANIMALS. 



(a) The degree of development of the 

 animal body, and the type of organization, 

 must be clearly distinguished. The degree 

 of development of the animal body con- 

 sists in a certain amount of heterogeneity 

 in its component parts; in diversity of tis- 

 sues and of form. The more homogeneous 

 the mass of the body, the lower the degree 

 of development. The fishes, for example, 

 because they have a brain, a cord and a 

 skeleton, and present clearly the vertebrate 

 type, are held to be superior to all inverte- 

 brates, and the advocates of the supposed 

 law of development wonder that the bee 

 and most insects with metamorphosis give 

 evidence of greater skill and a more com- 

 plicated life. In the bee, however, nerves 

 and muscles are developed to such a de- 

 gree that they difl'er from each other much 

 more than do the same organs in fishes. 

 Indeed the nerves and muscles of the 

 latter seem to be soggy with the water in 

 which they live. 



(b) By type of organization is meant 

 the relations existing between the organic 



