It begins by describing the "people of the sea" which breed like human. This, however, does not pose any conflict with science. Although Rashi explains the Gemara as referring to mermaids, which interbreed with people, the Gemara is actually talking about dolphins, as I discuss in Sacred Monsters. But after that, the Gemara gets much more problematic.
The Gemara then says that "Any species in which the male has external genitalia bears live young; any in which the male has internal genitalia, lays eggs." As a general rule, this is strikingly accurate, and the Gemara's rules are often only meant to be general. However, this section of Gemara is intending to give absolute rules, as evinced by the fact that the Gemara on the previous page names the bat as an exception to its rule that every lactating mammal gives birth to live young (ironically, the bat is actually not an exception to this rule). And as an absolute statement, it is incorrect that any species in which the male has internal genitalia, lays eggs. Whales, dolphins, elephants, giant anteaters, and hyraxes all have internal genitalia, and none of them lay eggs. If you want to avoid this problem by arguing that the Gemara is talking about genitalia that are permanently internal, then you run into a problem with the first part of the Gemara's statement, which would have to correspondingly be referring to species in which the genitalia of males are not permanently internal and can be extruded. The males of many reptiles, and even some birds, extrude their genitalia, and yet they do not bear live young.
The Gemara then states that if an animal mates during the day, then it gives birth during the day, and if it mates during the night, then it gives birth during the night. I haven't checked this one out, but I doubt that it's true.
Next, the Gemara states that any two types of animal that mate in the same position and have the same period of gestation, can interbreed. This is clearly not true. Countless species are identical in these details and yet are genetically incompatible and cannot produce hybrids.
Then comes a particularly fascinating passage:
Everything copulates front facing back, except for three species that copulate face-to-face: fish, humans and snakes. And what is unique about these three? When Rav Dimi came, it was said in the West: Since the Divine Presence spoke with them (Jonah’s whale and the primordial serpent). It was taught that the camel copulates back-to-back.
The Gemara continues to list the gestation periods of various animals:
The Rabbis taught: A hen [lays its eggs] after twenty-one days, and corresponding to it among trees is the almond [whose fruit ripens twenty-one days after its blossoming]. The [gestation period of a] dog is fifty days, and corresponding to it among trees is the fig. The [gestation period of a] cat is fifty-two days, and corresponding to it among trees is the mulberry. The [gestation period of a] pig is sixty days, and corresponding to it among trees is the apple. The [gestation period of a] fox and all kinds of creeping creatures is six months, and corresponding to it among trees is wheat. The [gestation period of] small clean animals is five months, and corresponding to it among trees is the vine. Large unclean domestic animals [go with young] for twelve months, and corresponding to them is a palm-tree among trees. The [gestation period of] clean large cattle is nine months, and corresponding [to clean large cattle] is an olive-tree among trees. The [gestation period of the] wolf, lion, bear, leopard, cheetah, elephant, monkey, and long-tailed ape is three years, corresponding to them are white figs among trees... The [gestation period of a] serpent is seven years, and for that wicked animal there is no companion [among trees].
So, what are we to make of all these? From a rationalist perspective, none of this poses any kind of theological problem. Following in the footsteps of countless Geonim, Rishonim, and Acharonim, we would simply say that Chazal were speculating or repeating ancient beliefs about the natural world that we now know to be incorrect.
But according to many (but not all) charedi rabbonim, such an approach is heretical. In particular, let us consider Rabbi Moshe Meiselman, because he is giving a shiur on this very page of Gemara in Baltimore tomorrow morning, at 5:50am in Kol Torah (it's open to all).
Rabbi Meiselman insists that any definitive statement made by the Sages cannot be challenged, and that to do so is heresy. Indeed, the bulk of his 800 page book on Torah, Chazal and Science serves to stress this point. He further repeatedly makes clear that he believes himself to be one of the few people (or only person) qualified to address these topics. So what does he do with the cases listed above?
Maybe he'd say that when the Gemara talks about camels and foxes and wolves, it doesn't mean camels and foxes and wolves, but rather entirely different animals that are unknown to us. That, of course, is an absurd suggestion, but is it absurd to suggest that he would offer such an absurd suggestion? Not at all, because that's exactly what he does in some of these cases!
Rabbi Meiselman claims that the atalef (bat) mentioned in the Gemara as laying eggs is not actually a bat, as has traditionally and universally been understood. Rather, he says that it is a platypus, which Chazal somehow knew about, and which they called by the same name as an animal that is birdlike in other ways, thereby misleading every single student of the Talmud for nearly two thousand years, other than him. And he claims (p. 5) that the snake mentioned by the Gemara is not any of the ordinary types of snake, referred to with that name in countless other passages in the Gemara, but rather refers to a different and entirely unknown species. So maybe Rabbi Meiselman would likewise claim that the camels and foxes and wolves and other animals mentioned on this page likewise do not refer to the animals that they were traditionally understood to refer to, but instead to unknown species!
However, Rabbi Meiselman does not actually do so, for reasons that are unclear. Instead, when he addresses one of the passages in this Gemara (regarding gestation periods), in a footnote on p. 6, he presents two possibilities. One is that the Gemara is not talking about the length of gestation, but rather "some other aspect of the reproductive process." This vague speculation does not seriously address the issues. What other aspect could be reconciled with these statements? What aspect of the reproductive process can be said to be fifty days with a dog, six months with a fox, and three years with a wolf? Furthermore, this does not address the other problematic statements on this page of Gemara, such as that any species in which the male has internal genitalia lays eggs, or that camels mate backwards, or that any two types of animal that mate in the same position and have the same gestation period can interbreed.
Rabbi Meiselman's other suggestion is that "the facts of nature have simply changed over the years." This claim (which is ironically often advanced by those who simultaneously argue that evolution is scientifically impossible) cannot be taken at all seriously by anyone even remotely familiar with zoology. Elephants used to lay eggs, but no longer do so? Countless species used to be interfertile, but are no longer interfertile? Camels used to mate back-to-back, but now awkwardly twist themselves around to mate front-to-back? Wolves, which are genetically virtually identical to dogs, used to have a gestation period of three years?! (Perhaps someone in Baltimore would like to attend the shiur tomorrow and pose these questions, and record his response?)
As I have stated many times, if someone is determined to believe these things, I have no problem with that, as long as they don't attempt to impose their belief on others. But Rabbi Meiselman claims that he is the greatest expert on these topics, and that anyone who takes the rationalist approach of Rabbeinu Avraham and Rav Hirsch is a heretic. And, amazingly, there are many people who take him seriously (albeit not outside of the charedi world). It's important to bring these cases of the Gemara to the forefront of discussion, in order to expose how his approach simply cannot be taken seriously. Any serious person knows that the rationalist approach of Rabbeinu Avraham and Rav Hirsch is not only not heresy - it's the only remotely reasonable approach to take.
(Note: The full index of critiques of Rabbi Meiselman's book is here: http://www.rationalistjudaism.com/2013/10/torah-chazal-and-science.html. You can download a PDF of all the parts written so far at this link.)