LOL! That's complete baloney.
Darwin never adopted Haeckel's "biogentic law," nor did most (or any scientifically prominent) evolutionists following. Darwin, indeed most scientists of the time period, adopted Von Baer's views which were directly contradictory to Haekel's. (See "Baer's Laws" in the link above, especially 3 and 4.)
Do your own text search of The Origin and other writings by Darwin if you don't believe me, and see for yourself who's embryological theories he cites. Haeckel never achieved anything close to the status of Von Baer wrt expertise in embryology, and the same is even more true for his "biogentic law".
How can "immeasurable dammage" be done by a theory that was never widely accepted, immediately challenged, and soon universally abandoned?!
More admissions of a fraud:
After this compromising confession of 'forgery' I should be obliged to consider myself condemned and annihilated if I had not the consolation of seeing side by side with me in the prisoner's dock hundreds of fellow - culprits, among them many of the most trusted observers and most esteemed biologists. The great majority of all the diagrams in the best biological textbooks, treatises and journals would incur in the same degree the charge of 'forgery,' for all of them are inexact, and are more or less doctored, schematised and constructed.
Francis Hitching, The Neck of the Giraffe: Where Darwin Went Wrong, Ticknor and Fields, New York, 1982, p. 204