Certiorari to review the decision and reso­lution en banc of the Workmen's Compensation Com­mission dismissing the claim of Julian Abana^[1] for compensation benefits under the Workmen's Compensation Act.

Stripped of non-essentials, the undisputed facts as found by Hearing Officer Pedro P. Pelaez of Labor Regional Office No. 4, and adopted by the Workmen's Compensation Commission, are hereunder re­told:

The respondent, Francisco Quisumbing, is the proprietor of the Dollar Taxi.  The petitioner, Ju­lian Abana, had been employed by him as taxi driver since February 13, 1952.  Before his employment as such, Abana was required to submit himself to physi­cal examination.

Sometime on August 21, 1958, the petitioner, upon arrival home from work, had a heart attack.  His family brought him immediately to the Philippine General Hospital where he was confined for five days.  His chief complaints were difficulty of breathing and chest pains.  The resident physicians who treated him diagnosed his illness as "congestive heart fail­ure, hypertensive".  After Abana had been discharged from the hospital, he rested for a few weeks at home, and when his health had improved, he reported for work again, informing the manager of the Dollar Taxi that the cause for his absence was that he had been stricken of heart disease.

Sometime in 1959, Abana had another heart attack while driving his taxi.  Since then, his heart ailment recurred very often causing him to be checked up and treated at the dispensary of the Philippine General Hospital from time to time.

It was established that everytime the heart ailment of the petitioner recurred, it prevented him from doing his task as taxi driver, but, however, he always reported the matter to his manager, asking either for sick or vacation leave with pay, which pay, unfortunately, was never granted him.  He was given some small amounts, though, in the form of vales.  Despite respondent's knowledge, however, of the al­ready weakened or diseased heart of the claimant (resulting to his permanent total and indefinite in­capacity for labor) and demands for sickness bene­fits, the former never gave the latter any benefits nor reimbursed him the amount incurred for medical expenses.

In view of petitioner's deteriorating health condition caused by his heart ailment, he could no longer endure to work as taxi driver.  Since Jan­uary, 1961, he had not been to resume work, nor could he perform any kind of work which entails phy­sical and mental fatigue or stress.

On the basis of the above facts, the hearing officer rendered judgment declaring petitioner's illness to be compensable under the law, and thereby ordered his employer to pay him the sums of P3,057.60 and P500.00 as compensation benefits and medical expenses, respectively; to provide him with medical, hospital, and surgical services and supplies until claimant's illness shall have been pronounced to be medically cured or arrested; and to pay office fees pursuant to the Workmen's Compensation Act.

As indicated in the beginning, the Workmen's Compensation Commission reversed the findings of the Hearing Officer and thereby dismissed the peti­tion.

The question presented is whether or not the petitioner is entitled to disability benefits under the Workmen's Compensation Act by reason of the heart ailment which he contracted during his employ­ment with respondent.

Apparently, the decision of the Workmen's Compensation Commission dismissing the claim for compensation proceeds upon the idea that the causal connection between petitioner's work and his ill­ness has not been sufficiently established.  There is no doubt that driving a taxicab for long hours in Manila and suburbs, including all its incidents, such as going through the congested traffic and helping passengers load and unload their baggage, causes severe strain and tension, and that it does aggravate a heart condition.^[2]  Since the record shows that petitioner's heart ailment occurred for the first time in 1958, while he was already an employee of the respondent, we make the conclusion that it was at least aggravated by his work.

While there is that possibility that factors other than the employment of the claimant may also have contributed to the aggravation of his illness, this is not a drawback to its compensability.  For, under the law, it is not required that the employ­ment be the sole factor in the growth, development or acceleration of claimant's illness to entitle him to the benefits provided for.  It is enough that his employment had contributed, even in a small de­gree, to the development of the disease.  It has been repeatedly held that under the Workmen's Com­pensation Law, it is not necessary for a claimant to carry the burden of proof to establish his case to the point of demonstration.  It is sufficient to show that the hypothesis on which he bases his claim is probable.^[3]

Viewing the case from a different angle, the exact medical cause of the illness, however is not really significant.  For, granting, only for argument's sake, that the evidence for petitioner is in­sufficient to show a causal link between the nature of his employment and his heart ailment, under the provision of section 44 of the Workmen's Compensa­tion Act, as amended,^[4] it is to be presumed that petitioner's illness, which supervened at the time of his employment, either arose out of, or was at least aggravated by, said employment.  With this legal presumption, the burden of proof shifts to the employer, and the employee is relieved of the burden to show causation.^[5]  We fail to find in the record any substantial evidence adduced by respondent em­ployer tending to disprove causal connection between claimant's heart ailment and his employment.  The mere opinion of doctors presented by petitioner as evidence cannot prevail over the presumption estab­lished by law.

The respondent contends, as he did in the Commis­sion, that the claim of petitioner has prescribed since no notice of the sickness or claim for compen­sation within the period fixed by law was filed by the claimant.  This defense is not tenable since the respondent had actual knowledge of claimant's ill­ness.  It is not disputed that "everytime the heart disease of the claimant recurred, it prevented him to do his task as taxi driver, but, however, he al­ways reported the matter to the management asking either sick or vacation leave with pay."  Under this circumstance, notice of injury or sickness would be superfluous.^[6]  Basically a social legislation de­signed to give relief to workmen, the Workmen's Com­pensation Act must be liberally construed to attain the purpose for which it was enacted.^[7]

As the amount to be awarded as compensation has not been the subject of dispute between the parties, We shall not disturb the findings of the hearing officer on this regard.

WHEREFORE, the judgment appealed from is hereby reversed.  The heirs of the deceased petitioner, who were named substitute in this case, shall be awarded the amounts and benefits in accordance with the de­cision of the hearing officer.  Costs against the respondent.

Reyes, Acting C.J., Dizon, Makalintal, Bengzon, Zaldivar, Sanchez, Ruiz Castro, and Fernando, JJ., concur.
Concepcion, C.J., on leave.

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^[1] During the pendency of this appeal, he died and his heirs were named as substitutes.

^[2] A. D. Santos, Inc. vs. Conchita Vda. de Sapon, etc., et al., G. R. No. L-22220, April 20, 1966.

^[3] Manila Railroad Company vs. Workmen's Compensa­tion Commission and Crispin Pineda, G. R. No. L­-19773, May 30, 1964.

^[4] "Presumption. - In any proceeding for the enforcement of the claim for compensation under this Act, it shall be presumed in the absence of substantial evidence to the contrary ?

"1. That the claim comes within the pro­vision of this Act; x x x "

^[5] Justiniano vs. Workmen's Compensation Commission, G. R. No. L-22774, November 21, 1966, citing Agus­tin vs. Workmen's Compensation Commission, G. R. No. L-19957, September 29, 1964.

^[6] National Development Company vs. Workmen's Compensation Commission, et al., G. R. No. L?21796, August 29, 1966.

^[7] Batangas Transportation Co. vs. Tomas Perez, et al., G. R. No. L-19522, August 31, 1964.