[In June 1861 a jotting in his notebook records that he is at work on the chick's skull, part of the embryological work which he took up vigorously at this time, and at once the continuation of his researches on the Vertebrate Skull, embodied in his Croonian lecture of 1858, and the beginning of a long series of investigations into the structure of birds. There is a reference to this in a very interesting letter dealing chiefly with what he conceived to be the cardinal point of the Darwinian theory:--] 26 Abbey Place, September 4, 1861. My dear Hooker, Yesterday being the first day I went to the Athenaeum after reading your note, I had a look at, and a good laugh over, the "Quarterly" article. Who can be the writer? I have been so busy studying chicken development, a difficult subject to which I had long ago made up my mind to devote my first spare time, that I have written you no word about your article in the "Gardener's Chronicle." I quite agree with the general tendency of your argument, though it seems to me that you put your view rather too strongly when you seem to question the position "that, as a rule, resemblances prevail over differences" between parent and offspring. Surely, as a rule, resemblances DO prevail over differences, though I quite agree with you that the latter have been far too much overlooked. The great desideratum for the species question at present seems to me to be the determination of the law of variation. Because no law has yet been made out, Darwin is obliged to speak of variation as if it were spontaneous or a matter of chance, so that the bishops and superior clergy generally (the only real atheists and believers in chance left in the world) gird at him as if he were another Lucretius. It is [in] the recognition of a tendency to variation apart from the variation of what are ordinarily understood as external conditions that Darwin's view is such an advance on Lamarck. Why does not somebody go to work experimentally, and get at the law of variation for some one species of plant?