These secondary Martians are bipedal, nearly  tall, and have "round, erect heads, and large eyes in flinty sockets"; however, their fragile physical structure, made up of weak skeletons and muscles, would have been broken by Earth's heavier gravitational pull.
The secondary Martians are bipedal, roughly  tall, and have "round, erect heads" with large eyes in tight sockets. However, their delicate physical structure, consisting of weak bones and muscles, would break under the influence of Earth's stronger gravity.