CONTENTS OF VOL. I. XXI 



Homology due to common descent— Exceptions due to spontaneous variation — 

 Eudimentary organs — Toes of Ungulata : leg-bones of serpents : wing-bones 

 of apteiyx — Comparison of these to fossils — Origin of these by descent — Ex- 

 ceptions to laws of adaptation, and of homology — Unity of type, a result of 

 community of descent — Problem of origin and modification of types — How do 

 we know that rudimentary orgaus are aborted, and not nascent ? — Classification 

 — Organs, if useless, must be aborted — Nascent lungs in lepidosiren. 



Pp. 241—251 



CHAPTER XXI. 



EMBRYOLOGY. 



Development is from simple germs — Species become unlike as their germs develop 

 — Development is diiferentiation — Embrj'os of higher foims resemble lower 

 forms — Development is indirect in most cases — Generally most nearly direct in 

 lowest groups — Process among the lower Invertebrata — They begin in the form 

 of Protozoa — Change of plan in development — Insect metamorphosis — Meta- 

 morphosis before birth — Insect larvae resemble lower forms of Articulata — So 

 of Batrachia — Larvae resemble immature low forms — Transition between water 

 and air-breathing Vertebrates — Branchiae of Crustacea — Circulation in verte- 

 brate embryo — Kidneys of same — Brain of human embrj'o — Development of 

 spinal column — Blood corpuscles, white and red : their threefold relation — Can 

 these facts be referred to the principle of adaptation ? — We do not know the 

 data for an answer — They are, more probably, records of ancestral forms — 

 Useless organs, and useless modes of development — Eudimentary organs in the 

 embryo only — Eudimentary organs largest in the embryo — Differentiation of 

 embryos — Groups of groups — Characters of the widest group appear first — Von 

 Bar's law — Characters of widest groups are least variable — Connexion of this 

 with Von Bar's law — Eeason — Mollusca, Articulata, and Vertebrata — Charac- 

 ters not embryonic are subject to exception — Unsymmetrical molluscan develop- 

 ment — Importance of embryonic characters in classification — Cirrhipedes : their 

 crustacean larvae — Dorsibranchiata, and iubieolce — Eeversion is sometimes the 

 retention of embryonic characters — Flounders — Fundamental and adaptive cha- 

 racters — Homology and analogy — Homological resemblances are fundamental — 

 Analogical ones are adaptive — Flounders — Constancy of fundamental charac- 

 ters, a case of the law of habit — Exceptions — Likeness of lai'val form proves 

 affinity : but not the converse — Insects — Beetles — Land salamander — Crustacea 

 — Fresh-water Crustacea undergo no metamorphosis — Direct development sub- 

 stituted for indirect — Laws of habit explaining nietamorphosis, and the loss 

 of metamorphosis — Batrachians — Land salamander — Young pigeons of various 

 breeds — Series — Fishes — Batrachians — Air-breathing Vertebrates — Descent of 

 the latter from fishes — Series — "Worms — Insects developed from worm-like 

 larvai — Insects directly developed — Crustacean series — Nauplius — Penoeus — 

 Other Malacostraca — Fresh-water Malacostraca have lost their metamoi-phoses 

 by variation — Series in Hydrozoa — Hj'dra — Hydrozoa ^\ith flower-like genera- 

 tive organs — Generative organs becoming detached as Medusae — Metagenesis — 

 Medusa producing Medusae directly — Parallel series — Summai-y — Nature of 

 metamorphosis — How lost — Objection — Reply. 



