The review: “Unravelling the heritage of lost traits” by Sadier et al. gives an interesting overview of the evolution of trait loss. While it is clearly not comprehensive - given such a broad topic - the review still covers most important aspects of the topic and I enjoyed reading it. I particularly enjoyed the part about the degree of structure loss and that complete loss without any morphological vestige is very rare.

I have only minor comments that are mainly meant as suggestions:

Page 4, Line 51: I would like to have seen Stemmer et al. 2015 (PMID: 25617433) cited and discussed here as well.

Page 5, Line 14: Either as part of this paragraph or as its own paragraph it would have been nice to see a stronger emphasis on developmental constraint. Maybe one of the reasons that certain structures cannot be entirely lost is because they are needed elsewhere. This is discussed in this paragraph to some extent, but a good example that would highlight developmental constraint can be seen in cavefishes that do develop an eye to only later degenerate it. The cells contributing to the retina and other parts of the forebrain are so highly intertwined that a development of a normal forebrain without an eye may be an impossibility. (as discussed here: Retaux 2013 and Krishnan 2017 (PMID: 24079393, PMID: 27994128).
Somewhere I would also have liked to read about Rajakumar et al. (PMID: 30305737) as an example of a rudimentary organ in development. To me this is one of the most exciting discoveries of the role a rudimentary organ in a different context in recent times and should be mentioned somewhere.

Page 6, Line 54: the gene is not completely lost, it’s function is, but the gene is still there for the most part. Maybe this could be changed to “inactivation” instead of “loss”

Page 20, Figure 2: Development is written in French and the reference “McGaugh et al. 2004” is not in the reference list.

The authors have carefully addressed all reviewer comments. I have no further comments.