30 Heredity. 



instance, has been slowly evolved from an ancestral rhi- 

 zopod, and since the ovum of a horse is homologous with 

 a rhizopod, or is morphologically equivalent to it, we 

 have in the gradual phylogenetic evolution of the horse 

 species from an unicelluhir ancestor, a satisfactory ex- 

 planation of the ontogenetic development of the indi- 

 "vidiial horse from an unicellular ovum. 



As soon as attention is fairly fixed upon the subject, 

 the weakness of this explanation becomes so evident 

 that I take the liberty of making tlie following quota- 

 tion from a well-known authoritv, in order to show that 

 the explanation hns been soberly advanced. In making 

 the extract from Haeckel's wa-i tings I am not actuated 

 by a desire to attack his views, for the same idea can 

 be found, expressed pretty definite!}', in the Avorks of 

 many other writers, and this particular selection is 

 simjily a matter of conyenience. 



Haeckel says : '^ Until recently the greatest students 

 of embryology, Wolff, Baer, Remack, Schleideu and the 

 whole school of embrj^ology founded by them, have re- 

 garded the science as exclusively the study of individual 

 development. Far otherwise to-day, when the mysteries 

 of the wonderful history of the development of individ- 

 ual organisms no longer face us as an incomprehensible 

 riddle, but have clearly revealed their deep significance : 

 for the changes of form which the germ passes through 

 under our eyes in a short time are, by the law of inher- 

 itance, a condensed and shortened repetition of the cor- 

 responding changes of form which the ancestors of the 

 organism in question have passed through in the course 

 of many million years. To-day, when we lay a hen's egg 

 in an incubator, and in twentv-one davs see the chick 

 break out of it, w^e no longer gaze in dumb wonder on 

 the marvellous changes which lead from the simple Qgg 



