936 ON THE SHAPES OF EGGS [ch. 



between the skeleton and the egg of various birds, to shew, for 

 instance, how those birds with a deep-keeled sternum laid rounded 

 eggs, which alone could accommodate the form of the young. 

 According to this yiew, that "Nature had foreseen*" the form 

 adapted to and necessary for the growing embryo, it was easy to. 

 correlate the owl with its spherical egg, the diver with its elhptical 

 one, and in like manner the round egg of the tortoise and the 

 elongated one of the crocodile, with the shape of the creatures which 

 had afterwards to be hatched therein. A few writers, such as 

 Thienemannf, looked at the same facts the other way, and asserted 

 that the form of the egg was determined by that of the bird by 

 which it was laid and in whose body it had been conformed. 



In more recent times, other theories, based upon the principles 

 of Natural Selection, have been current and very generally accepted 

 to account for these diversities of form. The pointed, conical egg 

 of the guillemot is generally supposed to be an adaptation, advan- 

 tageous to the species in the circumstances under which the egg 

 is laid; the pointed egg is less apt than a spherical one to roll off 

 the narrow ledge of rock on which this bird is said to lay its solitary 

 egg, and the more pointed the egg, so niuch the fitter and hkeher is 

 it to survive. The fact that the plover or the sandpiper, breeding 

 in very different situations, lay eggs that are also conical, ehcits 

 another explanation, to the effect that here the conical form permits 

 the many large eggs to be packed closely under the mother bird J. 

 Whatever truth there be in these apparent adaptations to existing 

 circumstances, it is oiily by a very hasty logic that we can accept 

 them as a vera causa, or adequate explanation of the facts ; and it is 

 obvious that in the bird's egg we have an admirable case for direct 

 investigation of the mechanical or physical significance of its form§. 



* Cf. Des Murs, p. 67: "Elle devait encore penser au moment ou ce germe 

 aurait besoin de I'espace necessaire a son accroissement, a ce moment ou . . . il devra 

 remplir exactement I'intervalle circonscrit par sa fragile prison, etc." 



f F. A. L. Thienemann, Syst. Darstellung der Fortpflanzung der Vogel Europas, 

 Leipzig, 1825-38. 



X Cf. Newton's Dictionary of Birds, 1893, p. 191 ; Szielasko, Gestalt der Vogeleier, 

 Journ. f. Ornith. Lm, pp. 273-297, 1905. 



§ Jacob Steiner suggested a Cartesian oval, r + mr' — c, as a general formula 

 for all eggs (cf. Fechner, Ber. sacks. Ges. 1849, p. 57); but this formula (which 

 fails in such a case as the guillemot) is purely empirical, and has no mechanical 

 foundation. 



